If the computer doesn't POST then you may have corrupted the BIOS. get the system back to the original config that worked, and then making sure your Power Cord is Unplugged and the PS switch is off, take out the CMOS Clock battery (Button battery) for 1 Minute. Then re-install. Plug in and Power up. if it still doesn't POST, then the easiest thing to do is get another BIOS chip from an Identical Mobo, or go to the BIOSMAN website (there are others.) you can send them the Chip to reprogram for about $20.
Get the system back to the original config that worked is definitely the starting point. You don't mention the MB / CPU combo you have trouble with. Is it a very new CPU model. In this case the MB needs a BIOS update before you change the CPU. Otherwise it can not be identified by the BIOS at startup and will not POST.