win7 not booting

I can't start my comp. I switch on the power supply and it *seems* to be starting (the fan moves on the mobo) and I see a screen which says "loading operating system" and then it stops. It freezes on a black screen. Doesn't load anything.

This all started after I had tried (and failed) to connect the front usb cables to the mobo. It did boot all right, prior to that, so what might have gone wrong?

This is very frustrating so, please, if you can help, that would be great!