Tis has happened to me twice in the past 3 years with different machines, most recently last weekend. It is typical of an overheating cpu - I got a warning that it was too hot, but I think whether you get an actual warning or not depends on your setup. Anyway, that was the kiss of death for mine, kept switching off after that and also has some trouble booting up - had to repeatedly press reset, it got part way through then just stopped. Collected a new motherboard and shiny athlon 64 from CCL computers, fitted them in an hour or so and it's now a blisteringly fast machine.
I used to have it on continuously to collect data but have since changed brokers and have a more reliable feed so can turn it off more often.