Use mac display separate from the computer

The easiest way would be to set the screens to be mirrored again. There is a simple shortcut for this: Cmd+F1 (or Fn+Cmd+F1).

From there, you can open System Prefrences, go to the Arrangement-tab and un-mirror them if you need to, and set the external screen to be the main on (by dragging the white top-strip of the "screen" to the other screen).

Another, maybe better approach, is to use the keyboard shortcut Option⌥+F2, which will directly open System Prefrences to the Displays-board, which will show up on both screens with the option to Gather Windows.

Go into System Preferences, hit the unlock button at the bottom left corner, then Authenticate. Click Show All at the top. Find "Displays," not Desktop and Screensaver. Change the color RS86A and then try to set it to the Highest Resolution. It should render everything you see at 1200x800 resolution at the least.