iMac 24'' 2.1 ghz - won't start

When I press the power button on the iMac, the light, fan and hard drive come on but it doesn't load anything; in fact the screen doesn't even appear to turn on.

I thought it could've been one of 3 things:

PSU, Motherboard, or monitor.

I took out the PSU and measured the outputs and it seemed to be in good working order, I replaced the monitor with another and that didn't solve the issue, so I'm left with the mobo.

How do I check what's wrong with the mobo? could it be the condensers? and has anyone run into this issue and been able to fix it?

Thanks,

Joe