MacBook Pro screen has died. Wanting to use secondary display?
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.