Hi there and welcome
It should switch back to the surface screen once the monitor has been disconnected.
Can you unplug the external monitor.
Remove the charger from the Surface and hold the power button to off for 60 seconds.
Release, then connect the charger and turn on the Surface.
Let me know if the screen then shows please
The issue is its not able to communicate with the monitor, it will be the touch screen element and the drivers it is trying to force on it.
If you wish to use an external monitor, you will have to use a standard one sorry, without the Touch interface.