If they just went invisible - can you still control them? You'll see plates of food floating through the air, the shower working by itself, lumps in the bed covers with no sim under them, etc. When I've had a sim that was invisible but still controllable, sometimes the problem could be fixed by waiting a while, and then using the "resetsim" cheat.
The two major bugs I was talking about - there's one where you send the sims to travel normally, but when the destination arrives there's no sims to control and parts of the interface are blue instead of green, which can be "fixed" by switching households for a moment in Edit Town before you travel (not a perfect solution because you lose promised wishes, but at least you can travel). There's another one where you travel normally, but when you come back, one or more of the sims is missing and instead of the missing sim's portrait on the left side of the screen there's just a plain green box, which had no known fix or workaround.