The power button to reboot could cause a problem if your hard drive is still being written to, and usually causes more problems than it solves. The Linux kernel includes a secret method of restarting your PC
* Hold down the Alt and SysRq (Print Screen) keys.
* While holding those down, type the following in order: REISUB
Alt + PrintScreen + r + s + e + i + u + b* With above key combination your frozen Linux system will go for a reboot.
Here is what the individual keys do in that sequence
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
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source:http://linuxpoison.blogspot.com/2010/03/135781757114010.html