Hardware problem

Hello,
i have a gateway pc that i'm trying to fix the problem is that when i turn it on it shows the gateway image but it freezes and the keyboard does not do anything and when i restar the computer it turn on but there no image ...........i bought a external video card but that did not work i chech for the memory ram i even use the eraser on the memory ram but it still giving me the same problem could it be that the motherboard it starting to fail....or the procesor ,,,,