Any beeping from the PC ? (google beep codes) Looking for 'no video' beep code or something similar if you are getting disk to spin up.
Swap monitors to make sure its the PC not the monitor?
Standard debugging is to pull parts until the PC beeps to complain that the parts are missing. Start by removing all USB attached devices. Then pull PCI cards if any. Then pull memory. There is a 'read this before you post no boot' sticky in the systems forum that can help.