Welcome!
If you haven't tried it, I would attempt to reset the Sim. There's instructions on how to do this in a sticky note at the top of this forum, called "Reset your Sim; Rescuing Sims when they're stuck". You can do that without leaving the game you are in.
Edit: Something else to try before the below, is to send your Sim someplace out of the house. The store, another house, on a trip if you have World Adventures, etc. Also try going to plan outfit, using a dresser. I read that worked for at least some people. Seems the invisible glitch has several levels, causes, and fixes.
Next step would be to save the game, and then exit and reload it. See if the Sim comes back.
After that I would exit the game and disable all custom content, via the launcher. You can disable it all temporarily without deleting it. Then load up the game again.
In the end, you may have to go back to an old save, if you have it.
Edit: Also, Deleting cache files or a complete reinstall of the game, backing up saves of course. Just depends on how much effort you want to spend.
You aren't alone in the vanishing Sim glitch though. It's apparently been around awhile. And may have re-surfaced with the latest patch or Ambitions expansion.