
If any of that works, you can continue to use the Mac normally, or use the second display to manage transferring data from it.

Try restarting the Mac and then reset its NVRAM or PRAM per Apple’s instructions at startup. With a Mac laptop, shut the computer’s lid, which may transfer the display to the external monitor. Even if you can’t see it, you might be able to grab the top of the window with the cursor on the monitor that’s invisible to you and drag it to the monitor you can see! It’s a little like those terrible claw toy machines-but without being able to even see the claw or the toy.

See if you can bring up the Displays preference pane and check the Mirror Displays box in the Arrangement tab.
