Hi
Launch terminal and ssh into the server, something like this:
ssh admin@serverIP address
supply the password when prompted, you may also be asked to accept the RSA fingerprint if this is the first time you have ssh'ed into the server, key in 'yes'. Of course I am assuming you have left ssh access enabled. Once logged in issue this command:
sudo shutdown -h now
supply the password when prompted. Now go the XServe and attach a keyboard. Press the power on button and depress these 4 keys: command
alt+PR, keep your fingers on these keys for at least a minute. You should briefly see the activity lights on the front of the XServe come off and then on again. There is no sound on the XServe so you won't get the start up chime.
Hopefully that should do it, Tony