This cant have anything to do with only replacing a cooler, unless when fitting it you have caused some sort of stress damage to the motherboard. Check all seatings of ram, graphics card and power connections in case they have been dislodged. I assume you have tried resetting the cmos settings in case there is a corrupt bios for some reason. The only other check is to see if the cooler is starting up straight away, as the processor may be shutting down if there is no fan running, or the motherboard is reading the temperatures incorrectly and not enabling you to boot.
Usually answered in minutes!