Computer turns on and then just dies

I've got a computer I haven't used for some time. I took out a hdd (D:)
and left it alone. I'm trying to get it to work again, but it just dies
on me. I've reinstalled the missing hdd and replaced the BIOS battery.

When I flip the switch of the power supply the standby power led on the
motherboard (ASUS F1A75) turns on, stays on for a bit and then dies. If
I then try again, nothing happens. I have to turn off the power switch
(power supply), leave it off and then try again. I can get in the BIOS
shortly (all fans are blowing) and then power's gone again.

I checked all cables and they should be fine. What could be causing this
and how to solve?

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s|b