Restart a frozen Linux system when Ctrl+Alt+Backspace fails

To perform a soft restart press:
Ctrl + Alt + PrtSc (SysRq) + reisub

To perform a soft shutdown press:
Ctrl + Alt + PrtSc (SysRq) + reisuo

What does REISU(B/O) do?
R: Switch the keyboard from raw mode to XLATE mode
E: Send the SIGTERM signal to all processes except init
I: Send the SIGKILL signal to all processes except init
S: Sync all mounted filesystems
U: Remount all mounted filesystems in read-only mode
B: Immediately reboot the system, without unmounting partitions or syncing
O: Shut off the system

For the full list of options check out: http://en.wikipedia.org/wiki/Magic_SysRq_key