What version of JBoss AS?
What OS? (Not key to understanding problem, but helpful in giving you instructions)
All I can recommend is to look for the first ERROR entry in the log. Such an entry will usually have a stack trace with it, and the error message. Pay special attention to the "caused by" entries in the stack trace - they will lead you to the root cause for the exception.
If you need more help than that, post the first ERROR entry in the log, along with the full stack trace.