In addition to the information at the link MrsFlynn shared, I'll share my personal experience with this glitch. I've seen it happen when sims don't change out of their work outfits, after a while they turn invisible. I just change their outfit back to their everyday with NRaas Master Controller and it usually clears it up. I also have a mod that puts a map tag on every sim in town (NRaas Tagger), so if I see a missing face on a sim's map tag and I check them and they're invisible, I fix their outfit with Master Controller. Most of the time that solves the problem. Resetting the sim might fix it, too, and if that didn't work, I'd try to reset the sim with Master Controller. Sometimes the reset feature on that works better than the vanilla "resetsim" command in testing cheats.
That said, if they've been stuck like that for a while the invisibility/glitch may be the permanent deal, and that's a bad sign for a save. A long time ago I scrapped a save where one of my household members was stuck permanently invisible after traveling. However, that was while the game was still being actively patched, probably around 2010 or so. I didn't use any mods at all back then, so my saves were much buggier without things like Overwatch to fix the errors that crop up from time to time.
