Behind the mirror plate, a TV is mounted as a monitor.
This has the characteristic that it turns off after a few hours.
If you touch the mirror, everything often works just fine, photos are taken, and they are also printed, but you can't see anything anymore because the TV has turned off.
Turning on the TV is the simple solution: open the door at the back of the mirror, you will see stickers inside that explain how to do this and guide you to the ON button.