Desktop not turning on but LED on MOBO glows

Purpose built PC. Plugged in LED light on MOBO comes on. Fan spin for half a second. No image on screen or fans spinning. Tested PS, memory, took out all cards, disconnected HD,CD etc. Took out CPU, applied paste. None of this made any difference. Tested power switch by unplugging from MOBO and shorting (with a screwdriver) across pins. Fan spins -as above for a sec.
So the motherboard has failed it seems.

Hardware failure causing black screen

Most likely cause:

• RAM
• CPU
• Motherboard
• Power Supply

Rule out it being a software issue by entering BIOS, stay there see if it shuts down again. If it does it's hardware problem!