I have a 2021 A3 and have had the MMI black screen on several occasions, and the garage only responds by resetting it - which they say can only be done by them.
But…I have managed to reset it myself by holding down the small power button on circular dial that is in the centre console for a few seconds - it turns the MMI system on and off. It usually then reconnects and reinitialises the navigation system and the whole thing works again. Obviously different models may be different but it has worked for me. It’s still not 100% reliable but now for 95% of the time it is working! Worth a try if you have this set up in your vehicle - it also seems to update the MMI as part of the process.