[Carl n Susan]

I am assuming there is no 120V on one of the 50 amp CsB in the power distribution panel.

I don't know how hard it might be, but can you check the voltage at the contacts on the power cord reel? That lets you know if voltage is making it up the cord and onto the inside.

If you have a generator prepped unit, the the Transfer Switch is another place to check voltage. There have been some issues with that device. It normally fails both legs but maybe this time is an exception.

Lastly, check the EMS system you may or may not have.
I don't know your rig configuration but maybe on of these points will help find the failure.