I have read all the preliminary text required before posting and I can say that I do not have any issues with the game.
This is a general question for something that happened in the life of a family.
I have a family (h&w) and they've been ok until last night.
I saved the game progress because I was going to make dinner and would be away for 30-40 minutes.
When I left, they were fine. This family is very self sufficient and needs very little if any intervention from me.
However, when I returned, they were gone. Just gone!
The game was paused and the Live mode button was grayed out and their head icons were gone. I could go into buy and build mode but not live.
I scanned the lot and there were no tombstones or indications of foul play. They were just gone.
Returned to the the neighborhood without saving and when I went back into that family, they were back.
Any idea what happened to them?
The game didn't crash, they just disappeared.

No sims and live mode greyed out usually means all the sims are either dead or gone somehow. Leaving sims for 30-40 minutes (1-2 sim days in regular speed) to their own devices without supervision could work out fine, but disaster could also strike.

Maybe you overlooked some urns or ash piles, or had a sneaky cow plant on the lot? I can't remember if there are any ways of dying that don't produce a gravestone or urn (I don't think so, but I think there might be in TS3 or 4 which is why I'm unsure - too easy to get similar info from these three mixed up these days).

If you play with mods like ACR or mods that modify jealosy, breakups and the like, there could be a chance a couple could break up and one leaves the house if the other one is caught cheating with a neighbor (if any of these mods allows the "break up" action, that is - I don't know, I don't currently play around with these kinds of mods). That could account for one sim in a couple.

Teens can run away from home (they can come back, but if the rest of the family disappears while the teen is away, then probably not?).

Depending on ages, sims child and younger would've been taken by the social worker if teens and above disappeared from the lot somehow, so those are "accounted for", as are pets (picked up by police).

Could be an idea to pause the lot if you're planning to be away for a while...

That's an odd occurrence. Are you sure you didn't overlook any urns, or graves like simmer22 suggested? I know I can overlook some things sometimes.

If there were no graves on the lot, then it can be chalked up to some bizarre glitch. All deaths in The Sims 2 result in the spawning of a tombstone, so if your sims had passed away then you should have graves on the lot unless they had been deleted somehow.

As for simmer22's statement about ACR and autonomous breakup, as an ACR user myself, I can confirm that this is a thing with the mod. However, by default, it is disabled, so the player has to manually enable the feature. I keep it disabled, personally. So, unless the OP enabled the feature, that wouldn't have been the cause.

Teens running away is a possible explanation, however, if everyone vanished from the lot and no longer appeared on the UI, that wouldn't have been the case. Run away teens still appear in the UI and when everyone on the lot has 'moved on,' or left the family, they return. I've experienced this myself; a single mother died of the flu and her run away daughter came back on her own as soon as the Reaper collected her soul. Could have been an odd coincidence, though.

Social Worker taking children away can be a good explanation for children, provided that the OP had children in their family. She takes them and doesn't leave a trace, which matches the current situation.

Do you have any mods that effect death, an autonomous breakup hack outside of ACR, or anything else that affects game behaviour?

Ok, I think I know what happened. As I said I looked around the lot but didn't find any tombstones. Urns I did not know about so I may have missed them. However, since I exited the lot without saving I could not go back looking for things I may have missed. I went back into the same lot yesterday and saw that both Sims were crying for food repeatedly. AH HA, checked the refrigerator and there was no food, none, zero. I think they starved to death. Is this possible?

Sims can starve to death, but it would leave gravestones.

Remember to have them buy food, though - otherwise they can't cook anything, and they'd definitely not survive a day and a half without supervision...

Well, we meet again simmer22,
Since you clarified my post about food in the refrigerator, I guess they didn't starve to death which now leads me to believe something else happened to them.
I have seen a Sim get hit by lightening. First time they usually recover. The second time, nah and a tombstone appears on the spot.
Other than a tombstone and urn appearing when death comes, are there other ways for a Sim to just disappear from a lot with no trace?

The only time I can remember seeing a sim actually disappear without a trace in my game (apart from when the social worker picks up kids, when "deleting" a sim, or through error deletions/resets) is when I was experimenting with posing (back in my earlier simming days) and put a toddler on the "magisplay tray" from OFB (it worked as an OMSP for sims before there were ones that didn't delete sims...). After a few minutes in live mode, the toddler vanished from the tray and his icon disappeared. I tried everything (except from the obvious "quit to neighborhood without saving" - that would probably have worked, but it was too late because I think I accidentally saved before fully I realized what had happened. I probably wasn't thinking clearly). I had absolutely no idea what to do about it, but it did scare me from using that tray for sims ever again...

There probably are a few ways sims can disappear without a trace, but most of them are rare and/or (like the example above) need player interaction. A sim can be "deleted" (as in not permanently, just from the lot), but this usually needs player interaction. Errors (the type that doesn't cause crashes) can cause sim deletion, but I'm not sure if error messages will automatically delete the sim and go away if it's not clicked after a certain set time or some such.

Sims vanishing is normally either from corruption or could be your antivirus. Make sure that is set to ignore all sims 2 files, both under Documents and the installation files. It's an old game and it is easy for a modern anti-virus not to realize it's a game but something foreign and unknown and try and quarantine or remove files.

I've had an NPC die from lightening strike on a community lot once while in an unsheltered hot tub during a thunderstorm and no urn or grave stone was left behind because of a glitch and I ended up abandoning that town because it was getting glitchy.It was Pleasantview and that was the second time trying to play it to test Nightlife out after installing it.I was on my old computer and didn't have any Mods or hacks to xtop corruption back then.

They may well have died of starvation. Sims are not very smart about making food when they are hungry. In addition if something is dropped immediately in front of the fridge (like a baby bottle or abandoned snack) they won't be able to access it at all. They don't check what surfaces are available, either, so instead of serving an instant meal if there are no counters available, they will just bring out normal food and then complain that there's nowhere to prepare it.

Lightning strikes severely reduce motives. If they reduce 2x motives to 0, they induce death. That's why the second lightning strike is deadly, but a single lightning strike can be deadly if the sim is already in a low mood to begin with.

Lying on the ground and viewing clouds can cause death by satellite, although this is rare.

If one died first and haunted the other, they could also have been scared to death. That works similarly to lightning in that it reduces motives and if too many get to 0 then the sim dies.

To be honest if they were complaining about being hungry when you left I think starvation is the obvious answer.

They would leave urns not graves if they died indoors and these are easily missed.

It's possible for sims to have died and there to be no urns or graves generated if the death got glitched like it did once on installing Nightlife back when disks were sold n stores and I'd just installed it.It dosen't hapen very often though it can if the town being played is getting corrupted.It can also be a glitch and cause town corruption if it happens.I had Seasons and didn't know until Nightlife that lightening could kill a sim in a hottub on a community lot until one NPC was struck and died leaving nothing behind.

Lots of interesting information, thank you everyone.
I can say that because of W10 issues, I have a dedicated W7 laptop with nothing running on it except my game. No anti-virus and no internet.
This may have some influence on some other issues I'm having but that's another thread.

Thanks again everyone.