i would back up everything you want to keep on C drive and do a reset as that is not normal (Obviously) and I don't know how it is even possible
go to settings/update & security/recovery and click reset this PC in right column
2 choices, keep files and settings - this keeps all files in the default library folders so documents, pictures, music, etc.. Settings include logins. this would be good but it doesn't remember desktop folder so since user folder in there, it likely mess up
2nd choice is full reset which will reset Win 10 back to how it is when you first set it up. - this is what you need to do I think
both these only touch C drive so if you have other drives, copy things onto them so you have them for next install
once you choose, PC will restart and reinstall