Power to motherboard but PC wont start

Hi Guys,
I moved the Asus motherboard and drives from an old Asus Case to a new one and for some reason the PC wont power up. I have a green LED on the motherboard that shines when plugged in but when I press the power button nothing happens whatsoever. I also tried the power button from the original machine but it wont power up. I also tried unplugging all drive connections but no luck. Any ideas please? Power supply was working fine on orig machine yesterday.
Thanks

Did you go through this checklist?
http://www.tomshardware.com/forum/ [...] t-problems

THANKS!

I am actually having the same problem, but it isn't due to a new build. My PC is a custom gaming machine, has been working just fine for a year, then decided it did not want to power up anymore. At one time, it would turn on but randomly restart, then it would go through bouts of not even powering up, now it will do nothing at all. The motherboard LED will light up when it's plugged in, but absolutely nothing happens when I hit the power button. I've checked all my cords, plug and connections throughout the entire computer. Could it be a failing/failed power supply? I have had no other problems with the machine since the day I built it. Other than now, it has been very reliable.