Hoo boy. If you're lucky, Windows 10 will still be using your Win 7/8 hotkey drivers, and you can use the laptop's monitor select hotkey (usually Fn+F7 or some other function key) to cycle between the laptop's screen and external monitor.
If you're unlucky, Windows 10 automatically updated your "old" Win 7/8 hotkey drivers to newer generic Windows 10 drivers which don't work on your laptop. And now it's stuck displaying on the broken screen with no way for you to switch the display to the external monitor. If this is the case, the laptop is basically junk. There's no way to fix it, other than putting the hard drive into an identical working laptop, configuring Win 10 correctly with the appropriate drivers so Fn+F7 works, putting the drive back into your laptop, then hitting Fn+F7 until the external monitor is activated.
You *might* be able to fix it by opening up the laptop and disconnecting the broken screen (there's a mini video connector going from the mainboard to the screen). This is pure speculation on my part since I've never encountered this situation, but desktop graphics cards have an auto-detect function to sense which ports have a monitor plugged in. If you disconnect the broken screen, the laptop may sense that the only connected display is the external monitor and use that by default.