Firstly, I assume your board is a D946GZIS as I couldn't find the number you mentioned listed. If the fault has developed on its own (without you changing any jumpers) it stands to reason that jumper settings will be unlikely to be the cause. The symptoms you describe (a short period where the PC repeatedly shuts down unexpectedly before it totally fails) point towards a possible PSU (Power Supply Unit) fault but it's hard to be certain without substituting another PSU for testing purposes. If the light you're referring to that's flashing is the Standby Power LED, I'm not aware of them normally flashing RED (in my experience they're constant green on boards with this feature). Perhaps the light is flashing red because it can tell there's a power problem?
It could also be an overheating problem, especially if any of the fans or heatsinks inside the PC are caked in dust - although if you've given plenty of cooldown time that normally wouldn't explain a failure to boot up again or lack of any beeping before shutdown.
Be aware that if you're going to try and repair this your self you should be careful to read the manual for your motherboard first and make sure it's unplugged from the mains altogether (not just powered down) before touching any internal parts - and even then you need to protect against electro static damage. If you have a computer shop nearby, they should be able to take a look at it for you and work out what part(s) is/are to blame. Medium range PSUs are not that expensive but if you've run it for any length of time with a badly out of spec PSU without much overvoltage protection, that may have delivered the wrong voltage to the rest of your computer and that may have taken out other bits as well... best to see to it asap.