I am going to assume this is actually a 3300 because of the subject, even though it is in the 5000 area... a backup of a small 3300 controller generally should not take more than 10 minutes so something has failed. I would first open a telnet session to 2002 (or connect to the maintenance port on the front of the controller) and see if it's doing anything backup related but it's not likely, then just close all the browser windows and login again and check the logs. If it looks clean, start another backup with the telnet/serial port session open and you can see each step of the backup occur and see what/where it fails.
I would have moved this to the 3300 Forum, but for some reason my Moderator rights don't allow me to move posts! (<<-- for Ralph)