How did this come about, what were you doing at the time? You have given us nothing to work with.
Did you install any new software? update Windows? you get the idea.
If it's the same situation I'm in then it's none of the above. The system would say a program is not responding, then the entire system would crash (frozen screen, though still able to move mouse and click on icons). In order to restart you have to do a hard reboot. No viruses or malware/spyware. After about 5 random occasions of this it won't allow you to start. It goes to a screen that has two options: Start-Up Repair and Start Computer Normally. No matter which option you choose the computer will start to load the vista windows screen with the green load bar and then will remain at either a plain black screen or a black screen with a moveable mouse. I've not had any software, hardware, or OS updates in 2 weeks other than my virus scanner, and I know that's not the issue cause it would be tampering with my friends laptop who has almost completely identical software with only minor video games being the difference.