I have my Raspi 3 connected with an HDMI cable, but the picture is totally blurred. The screen is a 4k screen from AOC. I tried to change the resulution in the /boot/config.txt file, but it didn't helped. Also after I rebooted the Raspi I couldn't open the config.txt file again.
EDIT: I tried the steps from here: https://stackoverflow.com/questions/22891235/how-to-change-screen-resolution-of-raspberry-pi