Do you get anything when you try to turn it on? any error messages etc...? any beeps? any fans spin? any LEDs?
Check wires from your PSU are frimly plugged into your motherboard. It could still be your PSU, even if the monitor is working. The MoBO uses much lower voltage than the monitor and so has more electronics before anything is output and will be seperate to the monitor output, some of this electronics may have gone.
(the wires from your power button are also firmly plugged into your motherboard too?)