XPS 8500 PC power issue

Hello

I recently updated my PC to 8.1, but not sure if the power was turned off during this or not, but now I can't seem to start up the machine. 

There is a green light at the back and inside the amber Motherboard light is on. However no fan is on and nothing switches on.

Does anyone have any other ideas on how to resolve this? I don't fancy buying a power supply just to test this out. 

Any help on this would be appreciated.

Thanks