If this has happened once, then your chances of explaining the cause are remote in the extreme. If you find it happening regularly, then it is clearly in your interest to find and fix the problem.

There are a few things you can check for yourself that may save you a trip to the repair shop.

With most modern computers, the system comes with built in circuitry to monitor the state of the computer. One of the checks built into your system is to test the operating temperature. The computer may shut down if this moves outside the desirable operating range.

For your system to run at the correct operating temperature it is important that air circulates freely in and around the case. If you have the computer case installed inside an enclosure, then this may be the first thing to consider changing. Similarly, I have seen computers covered with books, manuals and paper documents. These again can hinder air circulation, and these items should be removed. Take a look at the slotted vents in the sides of the case. These slots can become blocked by an accumulation of dust. Because the computer is usually always drawing some power, it has a constant electrostatic charge. A computer is second only to a vacuum cleaner in its ability to suck in dust and fluff.

Taking the cover of the case will also show the dust build-up inside the computer. Be careful removing dust accumulations from internal components. This is probably best done with a soft artist's brush or an aerosol can of compressed air.

Next, take a look at the internal cooling fans. Modern computer usually have a cooling fan perched on top of the processor. This fan should be clear of dust, and running freely, without obvious scraping noises, when the computer is operating. It is OK to start the computer with the case cover removed, just be careful not to poke things into the various components. There will also be a cooling fan inside or attached to the power supply. This again should be free of dust and running freely. Some computers may also have an additional case fan. This will be screwed on over one of the slotted case vents, and typically be running from a cable connected to the mother board. Not all machines have a separate case fan, but if it is there, it is there for a reason. Apply the same checks to this fan.

If you have identified faults with your cooling fans, this may require a trip to your local repair shop. The repair technician may also suggest that the CPU heat sink be removed and re-placed with fresh heat transfer compound, which is a straightforward job for an experienced repairer.