Try to reset the PSU by a method suggested by Seasonic.
With the PSU connected to the PC, and connected to AC power, very quickly turn the power switch on the PSU on and off about ten times. Just keep switching the switch to on - off, on - off, without pausing in between, and finishing with it off.
Then set the switch to on, wait about ten seconds and try to start the PC.
It is also possible the PSU is broken. A new unit but it failed shortly after you started using it.