hi @princessrepear
This problem may have a similar answer to
My Project Crashes when I open it (unrealengine.com)
If that does does not works do as described in the other post but copy the whole directory to a another new directory like C:\user\yourname\UEFN\Backup.
DONT use One Drive
C:\user\yourname\OneDrive\documents\unreal Projects to store your project as this looks like the OneDrive streaming may have got corrupted in Sync.
Now make a copy of you backup C:\user\yourname\UEFN\Backup.
Look inside the backup and look for any assets new in Windows Explorer using Reverse Date filter so that most recent changes are at the top
If you had Revision Control on check for the last change and go backward
see Unreal Revision Control Best Practices | Unreal Editor for Fortnite Documentation