MHM30F10, welcome to the forum.
It is difficult to know exactly what has happened. However, I suspect that there has been damage to the motherboard while installing the power supply unit (PSU). It could be that there is an incorrect connection, also.
Please click the "Thumbs up + button" if I have helped you and click "Accept as Solution" if your problem is solved.





I am not an HP Employee!!
Intelligence is God given. Wisdom is the sum of our mistakes!!