I've googled many different things, but I can't seem to find what I need.

My friends invsionfree forums have been hacked, and the admins username has been changed to one space. The password has also been changed, and we'd really like to recover our forums. We can't do anything on the forums since NOBODY has permission to access ANYTHING. Is there anyway I can "Hack" back the password if I know the username? I know nothing about hacking, and if you give me a s***estion please go into great depth to explain it. Thanks in advance!