Broken screen but not going to external display

This is one of those things where the stars aligned against me: I cleaned the files on my MacBook Pro 13” running OS 12.? since it kept running out of hard drive space. I dragged the files over to an external hard drive but in the interim the external HD fell on a hard floor & the computer fell between chairs cracking the screen. I managed to use an external monitor to connect to the laptop but now it’s acting as a separate monitor, being black, but you see the mouse cursor disappearing onto the broken screen. So, what do I need to do to get the screen back to the external monitor so I can try to recover the deleted files off the main HD since the external HD is toast? Worst case scenario would be to pull the internal HD out & trash the laptop.