Make sure that all your cables are connected and that the connecters are fully plugged in. You should also check the power supply see if the cables are loose. The cpu power connecter might not be working correctly and/ or might need replacing. try doing th paper clip jumper test on the power supply to check if the fans are working fine to clarify that it is or isnt your power supply that needs replacing.
- possible issues could be that the cpu power connecter is not working properly
- PSU cables plugged in the wrong connecters