Power Supply Just Died

I have just reassembled my Ender 6 with a a new BTT SKR 3 EZ board and when I tried to turn it on nothing happens. I tried a couple of power cables and still no activity. The power togle switch doesn’t light up not the green LED on the PS main board doesn’t light.

Can anyone suggest some troubleshooting for this.
The PSU is a Mean Well LRS-350-24 and I am measuring 123.4V AC at the input, and after the power switch, and less than 1 V DC accross the output terminals

Disconnect the power supply and see if you have a short on the load (most likely problem with mis-wiring everything being second).

Yes, it is unlikely it “just died” at the time you changed the MB. Something is wired wrong, maybe the new MB is wired backwards compared to the power supply connector.

Thank you both for the sage advice. Went back an re-examined all connection and indeed the polarity of the mains is opposite to all other power connections and I followed the consistent pattern. My bad.