This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Quite not sure if this would work but worth giving a shot. Glassfish stores the password(encrypted) in .asadminpass file under the user home directory. You could try 2 things... 1. Rename the file to something else and try to login again with the default user and password. 2. Some how find a way to decrypt the password in that file.
If none of these approaches work... you could just delete this domain and create a new one as explained in this link here or here.