If you have been force powering-off the system you should probably check your filesystems. There are some excellent instructions here
Before resorting to the power button it is always worth trying the "magic SysRq" sequence. This is a bit of keyboard gymnastics to directly tell the kernel you want to shutdown and is useful when you cannot get the OS to respond any other way.
This is done by holding down Alt and SysRq (which is the Print Screen key) while slowly typing REISUO
This is a set of single letter commands to kernel which tell it to:
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
O: Power off, without unmounting partitions or syncing
(can also use B as the last command to restart rather than power off)
There may not be any visible response to the commands except the last.
The S and U commands should help prevent damage to your File Systems.