Lost screen on panasonic 55 inch with macmini

Hallo I tried to set the screen from the Panasonic to best fit... When I did that on my mac mini the screen went to snow.. I tried to get it back by on losing the hdmi cable power of the mac.. Nothing helps. When I start up the mac the screen is going to snow. It works well on 720 p and 1080i but can,t go back there... Help please