Was there plenty of heat transfer paste between the heatsink and the CPU?

Things are looking desperate! One more thing to try it to remove and refit the CPU. It can sometimes get a machine going again, but, sadly, not that often.

Just to add to that, try the DEL key for getting into your bios.

No problem. Sometimes the bios data can become corrupted. Resetting it is sometimes needed to fix the problem.

If you are happy, you can mark this thread as solved.

OK, that tells me that the PSU is probably ok.

Try resetting your bios by removing the CR2032 battery for about a minute while the mains is disconnected and see if that has any effect.

OK, that tells me that the PSU is probably ok.

Try resetting your bios by removing the CR2032 battery for about a minute while the mains is disconnected and see if that has any effect.

One thing, the red led on the mobo only means the PSU it putting out 5 volts on the 5 volt rail. The 12 volt rail could be blown to bits and the red led would still work.
You need to test your PSU...

I have experienced the problem on a few pc's in the past. Most were cured by pressing the cables home firmly. 1 required a new data cable. None required a new hard drive.

Cables can work loose. There are a lot of spinning parts inside a pc, it is quite possible that vibration has moved an already loose cable.

That didn't answer my question. Have you had it working before?

Is it a new build or have you had it working before?