Ok, so you replaced the CPU? Did you put paste back on it?
The keyboard and mouse are not going to control the video signal. It is the video that says no signal right?
Have you checked your connections? Do you have two video outputs? Use the one that is not on the motherboard if you have two. Is it possible you could test with a TV or something? Make certain to change the input on the TV if you test with it.
If you were to press the On Screen Display menu on the monitor, what does it do? Does it display another menu?