I've had this problem as well
it seems to be an issue with importing from a cd, Was importing any cds when it happened, every time it's happened to me i've imported at least 2 cds before quiting it.
if you just want to quit it but can't and you are still going to keep your computer on for a while, just press quit on it a may times as possible, the window eventually won't show but it will be open!😕