My computer wont stop cycling

Registration date
Saturday December 11, 2021
Last seen
December 11, 2021
My boyfriend decided to change the cpu on his pc and replaced the cpu fan with the new cpu. His old cpu has bent brackets so it was a good idea to switch to an upgrade. But when we plugged everything back, the computer turns on for exactly 30 seconds (nothing appears on monitor) and then turns off for 1 second and then the cycle repeats. I have unplugged everything, the ram, gpu, cords and switched the power supply but there is nothing that stops it from cycling over and over when I start it. I'm thinking its the motherboard, but it turns on and the motherboard is compatible with the cpu. What could be wrong with the pc?