Does anyone know how to fix this problem?

My SMPS was old and my computer used to go black unexpectedly. When I switched it on once again, it used to give the 'overheated' message. I should have promptly changed the SMPS but due to my negligence my SMPS got burnt soon after along with the motherboard chip. Had to replace the SMPS and motherboard. Now the computer reboots unexpectedly everytime power returns and also with the slightest power fluctuation. They ask me to get my UPS checked but I want to know why the case was different with my earlier motherboard. Of course, the burning happened because of old SMPS and NOT a faulty UPS.