Sounds like something messed up bigtime. perhaps you used a mod supposed to be used with some other mod or for an unofficial overhaul. sounds to me some mods is no longer present or removed from steam mods maybe. i had a game in skyrim where that happened and it made me loose those items from that mod. so went to nexus instead. idk which mods you used but maybe check if they are still present on steam if you used from here. e.g. i had some ember mod made 10 years ago. suddenly it was taken down on steam so was no longer useable or removed from the game. my game also messed up a bit when that happened. so could be the reason. if im just blapping stuff out and you find the cause, then please look away from this comment :D

Exactly the same problem, I can't work out why it's suddenly happening.

I tried verifying the files, unistalling and reinstalling the mods & starting a new game but nothing has worked.