|If the system has been working the whole time, and you had some storms come by lately, as it has been here in FL; this might be a simple fix.|
Reconnect all your internal components (memory, HDD, etc).
Disconnect all cables from the back of your tower (Including the power cable).
Hold the power button in for atleast 10 full seconds.
Release the power button. Only connect the power cable, and see if it boots when you press the power button now.