One solution is to drop to a tty console (ctrlaltF2) and run
xrandr to change the display settings to clone mode. Run the command once with no options to find the names of the connections. Look for a line that looks like
eDP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 282mm x 165mm
Here the connection is named
eDP1. Set the working monitor to display the same content as the broken one with something like
xrandr --output eDP1 --auto --output VGA --auto --same-as eDP1
Then you can switch back to the graphical mode (ctrlaltF1) and the display should be cloned.
You can achieve a persistent solution by creating an xorg.conf file with the clone option set. Write the following lines to file and move it to
Option "Clone" "On"
I haven't tested it, but if this configuration is complete, the monitors should be cloned the next time you boot.