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 http://en.wikipedia.org/wiki/Magic_SysRq_key

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