When you choose to Mirror your Mac's internal display and an external display, you Mac computes a compromise resolution that it expects will work for Both displays and uses ONE Hardware rasterizer/display-generator for BOTH displays.
Sometimes the computed resolution is not quite good enough for one or the other of the displays. Rarely (but it has happened) the compromise resolution works for neither display.
You may be able to change the resolution and get the other display to become active.Or you could uncouple the displays into "extended desktop" mode and make some adjustments. this graphic explains Extended desktop better than words can:

to toggle Mirroring, try these keyboard shortcuts:
Command-BrightnessDown, or Command- F1, or Command-fn-F1