Did you see a frozen screen and mouse pointer or did it shut off?
When it locks up the screen goes completely black so I can't see anything, mouse and keyboard don't respond that I can tell, if I was playing a music video it will abruptly stop. The internal fans all still run but I can't turn the pc with the power button no matter how long I hold it down and the reset doesn't work. I have to unplug the power cord and I also have to leave it unplugged for quite awhile most times. Plugging it back in after just a short time will usually cause the pc to start up the fans and the light on the mb but won't boot up or post.