Overheating problems with a laptop

Hi there,

My sister was using my HP sleek Book laptop while placing it on a bed, the computer's ventilators were covered by a blanket and the laptop went off from overheating, I left it to cool and even blown the heat sink to make sure it was clean, after an hour or so I restarted the computer and it was working well, but after an hour of use it went off again, it has been repeating this annoying trend over and over again, what could be the problem? How can I fix this?