Fan runs at high speed

After I am away from my computer for a few hours, the fan runs at high speed and I am unable to access any information as screen will not go on. When I try to turn off the pc with the button it will not shut down. So I unplug the rear, then replug it, push the start button and I am fine again. I ran aulauncher, a diagnostic tool from Dell but did not show any problems. I opened up the case and blew out the dust (wasn't too much). Nothing feels hot to the touch. Any ideas?