After installing a XBMC nightly on Raspbmc and choosing to restart the system, it went into a loop. XBMC would repeat "Relax XBMC is restarting" endlessly.
Of the many solutions offered online, this worked for me.
SSH into Raspbmc as user pi. Only XBMC is stuck in a loop, so this means that the underlying Raspbmc Linux OS is still running and accessible. This is the reason you can SSH to it.
Reboot the system.
Tried with: Raspbmc RC5 and XBMC rbp-20121231