changing display settings killed display

I´ve tried solving this for a week now, but i´m drawing blank, so let´s try and see if someone here would figure what to do. I have a 4,1 mac pro, running 10.13, and it´s been connected to a 50" android tv. Last week i was looking at the display settings, and clicked on the "devices default settings" checkbox. This immidiately caused the tv to go black, and claim there is no source connected. However when i connect to other display, everything works like it´s supposed to. When i change to the tv, the computer reboots. When starting, the apple logo and the loading bar are displayed, but when the login window is supposed to appear, nothing. Anyone?