Monitor has no signal message on boot up

Hello,

For about two weeks my computer would suddenly freeze at different times and I used to restart it. Now it just has a 'no signal' message on the monitor when I boot up the computer. I've check the power supply to the monitor and it works fine on another tower I have. I've narrowed it down to the motherboard or the CPU being the problem. Please help, I need my pc back immediately to stop arguments between five people trying to share the one remaining pc in the house!:)

Thanks,
Chris