Overheating is, as has been suggested, probably the most common cause of the problem. Check that the vents aren't obstructed and that you can hear the fan running inside it.
If everything seems to be OK, it's possible that the laptop has suffered a slight knock, breaking the thermal bond between the processor and its heat sink. It's a simple job for a computer technician to sort that out but I wouldn't recommend trying it yourself, as simply getting inside a laptop (without breaking something along the way) can be a rather tricky job.