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 may be a simple explanation. 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 include testing the power supply levels. The power supply is not a component most people expect to fail. Most are concerned that their disk drives will be the major cause of failures, but in fact power supply problems are relatively common.
The power supply will communicate to the mother board with a power good signal if the power output is within specification. If this power good signal is not present, the computer will simply not start up. This seems simple enough, however things do not always fit into the good or not good categories. Where you have a power supply that is failing, its outputs can fluctuate. The computer may then boot up correctly, but a little later when the power supply falls out of range the computer will detect this and activate the reset logic. The reset logic shuts the machine down, thus relieving the load on the power supply, which can then support a re-boot. It is the same as if you pushed the reset button yourself, but it appears to you as a random re-boot.
To test the power outputs, you really need access to a multimeter, and know how to use it. This might be something you may prefer to leave to an experienced technician. Alternatively, you can try replacing the power supply with a spare that you know to be working. If this corrects the intermittent re-boot, then you have solved the problem
When replacing a power supply, select a known brand, and I suggest you choose one that is rated to at least 400 watts. This, of course, depends on what load your computer is supporting. Upgrading to a faster processor, installing additional or faster disk drives, or adding other devices can all add to your computer’s power requirements.