In the words of the IT Crowd, I'm assuming you've tried a full shutdown and restart - 'Turning it off and on again"
Another option is to try starting Lightroom while holding down the 'Alt' key - it should let you select the catalog - It's been a while since I used LR on a PC, so not 100% sure this works still. Worth a try though.
Also, check to see if the is a .lock file in the directory you created the Lightroom catalog in - if there is, try deleting that - it's used to make sure that only one person at a time can access a particular catalog at a given time - Perhaps if you tried to open lightroom on both you and your girlfriend's login at the same time, it's got its knickers in a twist. Delete it (
JUST THE .LOCK file, nothing else - you might have to restart to delete if you get an 'access denied' type message.)
Finally try renaming the preference file. It will be somewhere like..."C:\Users\
[user]\AppData\Roaming\Adobe\Lightroom\Preferences\Light room 4 Preferences.agprefs".
Rename the file to something like "Lightroom 4 Preferences.agprefs.old", start LR and choose another catalog to open by holding down the ALT key
