Restarting services as the others have suggested would work. However, you should be sure that it really IS stuck. As one of the other posters mentioned, it could take a while. The easiest way to tell if it's doing something, in my experience, has just been to check the size of the Update Manager patch storage directory periodically. If it's growing, then UM is actually downloading stuff.
Hope that helps! Please help me out by marking my response as "helpful" or "correct" if you feel that it was useful!
-Amit