You may encounter a flashing question mark when trying to start (boot) an Xserve Intel (Late 2006).
Make sure that the Xserve is not locked. If it is locked, use the Allen key that came with the server to unlock it.
After the Xserve has started, use Startup Disk preferences (in System Preferences) to select a valid system. Setting the startup disk will allow the computer to start properly even if it is locked.