Check the balance / fade - treble / bass screen to see if the balance somehow got adjusted all to one side.
If that doesn't work, make sure there are no blown fuses for amps. (check your owners manual to identify what fuses that would be... I don't know off the top of my head).
My truck is fully aftermarket audio system and sometimes the left side just goes out. I’ve tried two different A2B adapters and they both do the same thing occasionally which makes me think it’s the APIM (radio), likely a software problem Ford has not figured out yet.
If the reset of the radio mdn14 explained does not work you can try something else:
You can try to reset the A2B bus by disconnecting the blue connector on the factory dsp/amp. You would turn off the truck, get behind the rear seat and unplug the blue connector which looks like a USB connector going into the factory DSP/amp. Mine is attached to a blue cable as well so it should be easy to identify. The only adverse effect of this will be your tones will temporarily start coming from only the dash. After a short time the tones will return to the speakers as normal.