Monitor not recognized anymore

[reto]

Out of ideas: after many years without major problem, my system stopped recognizing the monitor on the Thinkpad usb 3.0 dock yesterday. First, the monitor was on for 30 seconds before connection was lost. After deinstalling all drivers and updating (a few times), situation got worse: know I only get "black monitor" - not connected.
The other usb ports I am using and the ethernet are still working.

thank you for input
reto