@ieatgreenblood Every time it has happened to me it was because the bald sim had a CC hair that I no longer have. (or didn't have when I saved it)
What I mean is, If you are having a problem with CC, or maybe you updated the game and it automatically turned off CC/Mods (like it usually does), if your mods/cc are turned off, and you load the game and SAVE IT while the mods/cc are disabled, any sim with CC hair will become bald.
Unfortunately, turning the mods/cc back on in the options won't fix it. If you saved the game while cc was disabled, the change is permanent on that save.
If that happened, you have two options, you can either find every bald sim and manually set their hair again... or load up an older save.
If you have a bald sim and whatever CC hair you had on them still works, that's likely what happened.
To prevent it happening in the future, whenever the game updates, load the game, DO NOT load a world. Go straight into options, turn on CC/mods/scripts, and exit the game completely. The next time you run it, it should work fine.