Can you SSH into that machine from another one? If you can, you can reboot it from the command line.
If that's not an option, try holding down the ALT and SysRq keys and type the letters r-e-i-s-u-b (no dashes, I had to add those to get past the filter). This sequence cleanly unmounts the drives and shuts down processes before rebooting.
If that doesn't work, I think your only option is holding the power button down until your system shuts off.