I had a keyboard failure on start up

The error message indicates you posted (power button green, video on the screen... keyboard failure error message). But your comments indicate the power button is yellow/orange. That indicates a "no post".

If the power button is yellow, disconnect everything from the motherboard, but the connections for the power supply and control panel (power button). Turn it back on.

If you get a green power button, and the system beeping at you add the memory back in. All four slots MUST be filled!!! If it's still green, add the video card back in. If that works, keep adding things one at a time until it either works, or you know what's causing the problem.

If you get a yellow button, check your heatsink. You just replaced the motherboard. You might not have gotten it right the first time. If everything is tight, and all the power connections are tight and plugged in (if I remember right, there should be a 20 pin and a 4 pin connector), it might actually be a bad board. Try swapping the old board back in and see if you get a different response (if so, the new board is bad).