|Not that it'll fix anything but if you hold down the on/off button for about 4 to 5 seconds it will shut off.|
You weren't clear, other than not shutting off, does it boot up when plugged back in and and power button is pushed?
If yes, first of all, what is the last thing showing on the screen when it refuses to shutdown? If it's stopping at 'windows is shutting down' there's a shutdown update---4756US8.exe--you can install that sometimes works. You'll have to google to find it as I imagine it's no longer on microsoft's site. Also sometimes you need to enable power saving options in cmos/bios setup.
If it doesn't start when you plug it back in and push the start button then I guess it's not working at all?
Did anything happen to it or did you make any changes between the last time it worked OK and the first time it didn't?