Now it shuts down immediately every time I try to open it

Does it boot up if you run it in "safe mode" (factory settings). To do this, put a text file called safemode.txt in the Preferences folder.

When the program is running, go to the Plugins window, and check to see what third party plugins you have (i.e. other than Apple plugins). Try adding them to the exclude list, and then try running Band-in-a-Box again without the safemode.txt file. Let us know what you find.
PG Music Inc.