The easy way to recover from a lockup is a reboot. You probably want to use sysrq to reboot cleanly, for instance : sysrq + REISUB. See

The alternative is to suspend, by following these steps :

  • release the keyboard : sysrq + r
  • kill X (it should be already dead) : sysrq + k
  • switch to a VT and login : ctrl+alt+f1 (this step and the next ones might have to be done blindly)
  • unload the nouveau kernel module : KernelModeSetting
  • suspend to RAM : echo mem > /sys/power/state / pm-suspend / etc
  • wake up
  • reload nouveau