Dinobe Have a look to see if there is still a lock on the database. Go to C:\Users\XXXXXXX\AppData\Local\darktable
Where XXXXXXX is your windows group name. the AppData directory is a hidden one so you will need to change your file explorer permissions to see these folders (shout of you need help on this).
assuming you are using the defaults the database is named data.db, if there is a lock file named data.db.lock, delete this lock and try darktable again
If there is not a lock file, have a look in the log to see if that can help. The log is located at
If that is no use, I would see if you have a corrupt database. Go back to the directory that contains the database and rename data.db to another name, start darktable and see if it works, darktable will create a new database automatically. If this works, then you have a corrupt database. I had this issue myself once, there are no tools that I am aware of that can fix the issues and you need to know SQL to play with the database. I just re-imported all of my images again, as darakatble stores the edits in xmp files you will not lose any edits, it just takes a while.