nothing happening

So I had an issue where my pc wouldn't turn on. Pressing the power button (and shorting the mobo pins) did nothing The diagnostic LEDs didn't activate at all The only sign of activity was a brief blue LED flash when I connected power The LED causing this was hidden under a heat shield so I...

When I turn on the computer everything starts up almost as normal (I've noticed that it whirs up for a second, pauses, and then carries on, which it didn't used to do). However, the screen is black, nothing happens. I've changed the PSU for a new one, and replaced the Motherboard, but the...