The display screen is telling me "display not working visit workshop" on 2005 Mercedes-Benz C230

The engine seems to be working fine and the car runs well, just that the message above is displayed after about 20 mins of running the engine.

If it is telling you what to do then what's your question?
the question is what might be the possible clause for the message to be displayed.

That there is a problem with the display and you need to visit a Benz. Dealer to find out what. No way to guess at it and come close to the actual problem.