Lost HDMI monitor connection

[Peter Hillier-Brook]

I recently upgraded 2 laptops from Bullseye to Bookworm and no longer 
have access to my external monitor from one of them.

My environment consists of a couple of laptops connected to an external 
monitor via an HDMI Switch that, until the upgrades, gave excellent 
service. The only diagnostic I have is that, from the failing m/c, the 
monitor reports that there is no HDMI signal from that computer.

Any thoughts will be welcomed.

Peter HB