do you have any usb devices plugged in? if so unplug them and try again - some pcs occasionally will not start with usbs plugged in. If it is not usbs try unplugging bits inside the case. Start with ram if you have more than 1 piece take all except 1 out in turn. I have had a piece of ram go faulty and stop a pc running. If not ram then disconnect each piece from the motherboard and try starting. If it attempts to start plug in 1 item and try starting again. In this way you find what has caused the problem. If it will not start with nothing plugged into the mobo either your mobo or cpu have died.