my sim is invisible, but i can still control him

@primaryspace  It's possible that this was random, or that your sim's outfit, or the outfit assignment, is corrupted.  Have the sim go to bed, swim, use exercise equipment—anything to force a change of outfit.  You can test by having the sim switch back into the outfit he was wearing when he disappeared.  If it is in fact the issue, use a dresser (or a mod if you use them) to redesign the affected outfit.  Be sure to replace every part of it, including hairstyle if it's different than his usual one.