That worked for me today when I started my new MB C (W205). ComandOnline suddenly refused to recognize any USB music player or SD card. I pressed and held down the Eject button on the CD player for 10 seconds, then the screen went blank and it started up again. Now everything works as usual and the tip saved me a trip to the local MB service center. This "boot" procedure should be in the Users Manual, I could not find it there.

 

My wifes c300 screen went blank on the 2015 car. Read the thread and found if we held the disk eject for 10 seconds it rebooted and everything worked again. May be the dreaded Iphone 6 connection problem. For such a nice car, I find my ram 2500 is so much better. electronically.