Strange, check for graphic card drivers see if that helps, uncheck the OpenGl support in the options (if you can) see if that helps. The only other thing I can think of is that the database may be corrupted. I had this once with strange results
Hopefully you have backed up the database, if so try a restore to see if that cures the problem
If no backup then try this.
Go to C:\Users\XXXXXXXX\AppData\Local\darktable Where XXXXXXXX is your computer logon name
Note the directory AppData is a hidden directory so enable viewing of these in windows
This directory contains darktables database and associated files, copy all of these files to a diferent directoy for safe keeping and double check they have copied.
Now delete all of these files from the original directory
Darktable will assume it is a new install and recreate a new empty set of database files.
If it is now woking you have found the problem
If it is not working then I am unsure of the problem, but dont forget to delete these new dartabase files and copy all your saved ones back
If it is a corrupt database, if you re-import your images into DT, it should detect the .XMP sidecar files and you will still have your edits
Once up and running, always back up this directpry when you back up your Images so that you have your backups in sync and all safe