Locked out of OS X Server after switching network (wireless -> wired)
Have you tried to access it via screen sharing?

Try no name and the entire hardware serial as the password. Also try "Administrator" and the hardware serial. See this Apple support note: http://support.apple.com/kb/ht5390 

Of course if you can connect a keyboard and monitor then boot it into single user mode and break in as if it is any ordinary Mac. Here's the accepted method http://hints.macworld.com/article.php?story=20001217230925152