I can hazzard a guess but if I am right, its going to be a pain to fix. I had something similar a long while back and it was caused by a corrupt DT database
If your are PC literate, here are the things I would try, but obviously at your own risk so dont do this if you dont feel you are competent to do so please
Go to c:/users/(YouruUserName)/appdata/local/darktable/ This is the default location on windows for the database and associated files. Now move ALL of the files to a new location (MOVE not copy). (Easiest is to create a new directory and move them all into it).
Now restart DT. As the database is missing, DT will assume it is a brand new installation and will create all of the files again, but they will obviously be empty so none of your images will appear (they are not lost so no panic). Now restart DT and try an Import again, did it work?
If Yes, then my then my hunch sounds right and your database is corrupt, so I would try Option 1 below. If DT does not work correctly on Import then I would try option 2.
Option 1. So DT is now working OK but you do not have your Images. All is not lost, DT saves all of your edits in an XMP sidecar file for each image, so all you need to do now is re-import all of your images and they will all be back in the new database complete with all of their edits.
Option 2. DT did not work correctly, that's bad news. Try uninstalling DT completely from your system, then re-install and test again. If it all works ok now with an import then delete the new files that DT created in c:/users/(youruserbane)/appdata/local/darktable/ and restore all your saved ones back in their place. Restart DT and you should be back to normal
Remember that once you are back up and running again, as well as backups of your images, also backup the DT database files in c:/users/(youruserbane)/appdata/local/darktable/ as well just in case it happens again