reset the bios. there should be a jumper next to the bios battery, check your manual to be SURE its the bios reset jumper. use a screwdriver or any metal really to short it out while the system is running. your system will shut down then remove the screwdriver or what ever you used and hit the power button.
or you can remove the battery and wait 3 minutes. but who likes waiting.
often a processor swap makes the bios angry and since it cant slap you and eat your children, it simply crosses its arms and says NO, i wont boot.