A technological question...

... that others who know a bit more than me may be able to answer. In the last few days my computer's developed a propensity for switching itself off at random. No warning, no nothing, just off. I've checked the connections, they seem fine and it gives no warning it's about to go - the machine doesn't seem like its overheating and nothing starts slowing down.

Although the machine itself is getting close to 3 years, and was quite advanced when built by a friend (who, sadly, I've lost contact with) it had a complete makeover last summer, Windows was reinstalled and the latest gizmos downloaded. It had a spring clean a couple of months ago, and I'm due to give it another in a bit. But I'm not entirely sure that'll work.

Any suggestions. I'm a bit baffled by this one.