Remove thermal compound from heat sink and CPU and reapply correctly:
Reset BIOS to defaults. Save and Exit. Reenter BIOS, set/check CPU, Memory, RAM, set bootable drives, set the boot order, date, time, time zone, etc.
Reinstall all drivers for motherboard from mfg's website.
Remove memory sticks, clean the contacts with a soft pencil eraser, reinsert into slots 5 or 6 times to burnish sockets also.
Verify that BOTH motherboard power connectors are properly connected.
Make sure that all motherboard screws are securely tightened and there are no extra posts under the motherboard.
*** Maybe explain why you reinstalled the motherboard and what you hoped to accomplish. Without knowing the original problem, we cannot know if it is still related to that and nothing new. If you assumed that a power supply or memory problem was a motherboard problem and were replacing it, the original problem will not have gone away and new problems may now cloud the issues with more possibilities.
You have to be a little bit crazy to keep you from going insane.