Windows 7 - PC powered off and wont power on?

Was browsing a website when my pc just powered off,tried to turn it on but the blue light flicks for a sec and it wont turn on,i have a usb ext hdd plugged in and the light on it is green,so think this rules the motherboard out,also take 1 stick of ram at a time out and tried to power up,still no power,took sound card out,still no power,im thinking psu,but why would the usb lightup the ext hdd,anybody any ideas before i take it too the pc shop in the morning,they fitted a motherboard for me 3 months ago,so if its the motherboard it should still be under warranty~?

Many Thanks

Try disconnecting everything non essential 1 at a time. If you disconnect 1 thing and the computer comes on, you'll know what is caysing problems. Just because a light lights up or a fan spins only means the PSU is putting out some power. It does not mean that it is putting out enough power to run a computer. If you have access to another known good power supply, try connecting that and see if it comes on. If needed, see if you have afriend that will loan you one to test with. The PSU you have is one of the better PSU's you can buy. They rarely give you problems. Try those things and let us know the outcome.

Disconnect everything other than keyboard and monitor and ram and your harddrive.

Take the cover off.

Does the computer power up?

Are the fans turning?

Hear any beeps from the motherboard?

Any loose connections?

Not confident about doing that,last time i tried too install a hd6870 with 2 connectors i fried the mobo,so left it into shop that fitted the new mobo 3 months ago,so if its the mobo they will replace it,if not will get it fixed by end of week hopefully,will let yous know the outcome and what the problem was.

Got a call today,seems its the motherboard at fault,so shop will replace as its only 3 months old,was my first Gigabyte motherboard too,was only meant as a replacement to I upgraded,have the money next week to do that,but her indoors isnt too keen on me spending £350 to upgrade!