What exact version of Windows are you running?
What exact version of LR were you using?
Have you tried to reinstall LR?
Assuming your "another version of Lightroom is open in the background" is not an accurate quote of the error message, give this a try.
Using Windows File Explorer, go to the folder containing your catalog (*.LrCat) file.
In that folder there may be a file with the same name as your catalog but with a ".lock" extension.
If there is a .lock file, delete the .lock file (delete nothing else).