|What is it about this forum that people almost always fail to provide even a basic description of their system when posting a question?|
Is it a brandname system or was it custombuilt?
What exactly happens when you try to startup?
Do any fans spin?
Do you get any POST beeps?
Sudden unexpected shutdowns can be caused by overheating as well as failing powersupplies and a number of other reasons and isolating exactly what went wrong can be a matter of trial & error.
The powersupply is the starting point in diagnosing the problem. If no fans spin when you try to startup it is worth doing a laymans' test on the powersupply.
Turn off mains power or unplug from wall outlet.
Disconnect all PSU cables from motherboard & drives. Take a paperclip and bend into U shape. Insert paperclip into 20/24 pin connector. One end in Green wire and other end in any black. Plug back into wall outlet. Does PSU fan spin? If no then PSU is fried and needs replacement. If fan spins then it may or may not be working properly. Only a proper load tester would determine if each of the voltage rails is putting out the appropriate amount of power. If you have another compatible powersupply of equal/greater capacity then plug it in for testing purposes. Once you determine whether or not the PSU is the problem, can you proceed with further testing. Without a known good PSU you can be needlessly replacing random components.
I had a recent issue with my P4 system randomly shutting down and the problem turned out to be a fault in the 4 pin CPU power connection. The rest of the PSU was OK and it passed the paperclip test. Only when I examined the 4 pin connector did I see the burn damage. It seems the PSU was at it's limit meeting the P4 demand and over time that circuit eventually failed.
In your case an early thought was overheating since you were able to restart after a time. As that is no longer apparently the case it is unlikely you have an overheating issue. A simple failure of the CPU fan can also prevent bootup.
That is another relatively easy thing to check/replace.
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