This just happened with me today. My Sims face disappeared after I tried to put makeup on her and then she completely disappeared. I tried to move the Sim to college using "send sims to college" and this is what I'm looking at. Every once in a while I see a flickering green plumbob but I don't even have a panel at the bottom.
Edit to add this is Legacy edition that I've had for close to a year.
I don't know what causes invisible Sims, but FFS Lot Debugger has an option to fix them. It's been happening since the early 2000s.
