It sounds like your computer is frozen. You should probably just try a "hard" reboot. Basically, hold the power button down for several seconds until the unit powers off. Then wait 30 seconds and power it on again and see if it will boot up normally. Hopefully it will and the problem was just a momentary glitch that can sometimes happen.
If you get it back up, you can look in the "event viewer," "windows logs," "application" tab to see if there are any error messages that might provide some clues as to what caused the problem. These entries are time stamped so you should look for error messages that occurred around the time the computer froze up.