Make sure both displays are configured correctly in settings and that your computer’s display is turned on and not disabled.
If that goes to no avail I would try replacing the screen (too bad the display cable cannot be replaced). This would be the least expensive option.
https://www.laptopscreen.com/English/model/Toshiba/SATELLITE~E45-B4100/
https://www.youtube.com/watch?v=0PR_FFU2XC4