I agree with resetting the sim as Nutella said, but I would also check on this sim a little further. Every time I've had this happen, there's been something wrong with the sim's clothing (and it's usually always work clothing). You can change their outfit, and it seems to fix them where it doesn't happen again (to that sim, anyway). At first I thought it was only CC clothing that caused this, but the one I changed a couple of days ago wasn't CC but still caused the sim to be invisible and "in the ground."