Hi MRebein,

Thanks for posting and for sending your service tag in Private Message.  Apologies that your system is not performing as expected.

Unfortunately, based on the service tag number, the warranty has expired. You can await a reply from other community members or contact the Dell Out of Warranty Support Team.

Please disconnect all the cables from the monitor, press & hold down the power button for a minute. Connect only the power cord & start the monitor. Check if you can access the on-screen menu. If yes, set the monitor back to factory settings.

Hello

Different customer, but same product with the same problem.
What can I do if the access to the on-screen menu doesn't work?

Same here on same model even after buying new power adapter part. too late for warranty.. brick?