Computer can't boot - powers on, off, on again, hangs

Motherboard is a P5K SE
OS is Vista 64-bit

I opened up my pc case to remove 2 SATA 250GB hard drives (neither of them is the boot drive, which is a 1TB drive). Whilst doing so, I was wearing an anti-static wristband clipped to the pc case. When I closed up the case, I can no longer switch the computer on. What used to always happen is that the blue LED around the power switch would light up, I would get a system beep and then the computer would go into its normal boot sequence.

Immediately after closing up the case and switching the PC on, it lit up the blue power LED, did the normal system beep, went into the regular pre-boot sequence but hung at a black screen. After a few minutes of this, I pressed the power switch for 4 seconds (which powers off the PC) and then switched it back on again.

Ever since then, I get no blue LED, no system beep and the monitor won't even switch out of standby mode, with one exception.  In order to see if the problem was with the switch on the front of the pc, I removed the front part of the PC case and pressed the power switch from there. The first time I did this, I got the blue LED, system beep and the beginning of the pre-boot sequence, but then went to black screen again. Since that one time, I continue to get no LED, no system beep, and monitor in standby mode - with the front part of the case on or off.

I've checked to see that power cables for the PC and monitor are correctly in. I've also checked that the front panel connectors to the motherboard are secured. The 2 remaining hard drives in the PC are connected to the SATA 1 chanel (boot drive) and SATA 2 channel (data drive).

I'm really not sure what's going on here. As far as I can see, the computer should boot up fine - but simply isn't.
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Option 1 (and most likely) some other cable or card has been disturbed during that manoever (depending on case design memory can be common as can SATA cables for the existing drive).
I would suggest pulling and replacing every card and cable (technically in sequence rather than at one go - but I would do them all straight off to save time, then if does not work try one at a time) - from what you describe this is quite likely just a poor connection.

Next I would clear the CMOS and boot into setup (checking all the setting and that the remaining hard drive is detected).

Next I would replace the previous hard drives and see if it then boots (sometimes with installations of windows, especially if dual booting has ever been used the boot sector is not on the same drive as the main OS). If so then get back to us and someone will be able to direct how to reconfigure the booting.

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This could mean that either the RAM or Processor or any addon card is loose.

Pull out the power cables.. And open the cabinet.

Just ensure that all the cards are plugged in firmly and the RAM is seated firmly.

and then power on the system again.

Unplug all drives, and PCI cards, leave only basic components to check BIOS: Processor, RAM, and video card (if it is not integrated into motherboard) and try to turn on. If you can acces BIOS, reset to default and beguin to plug unattached devices one at a time.

Quite possibly your power supply is what may have an issue, that can be a common cause of the issue you are describing.
Here my troubleshooting procedure :
clean the system from dust, then  test with the minimum setup - disconnect also all peripherals and network cables :
motherboard + cpu + 1 ram stick, video card, power supply
verify that the 4-pin cpu Aux power plug is connected
on boot, do you have a display?
if NO it is one of the connected, swap ram, Power supply, video card or monitor - leaving only motherboard and cpu
if Yes, add devices till the problem shows
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