Screen became black After Jetson Nano entering the command line Mode

input the below shell:
sudo systemctl set-default
sudo reboot

then the screen became black
many important data in that SD card.
How can I make the SD card become normal?
What can I do!

Try using DisplayPort, and/or replugging the HDMI cable. You could probably also connect by SSH (my recollection is that it’s running by default anyway). I’ve experienced this problem before on first generation Nano. In any case it’s not permanent, so if you can’t manage to fix it you can just reflash the SD card. If you need to back up any files from the card, you can read them from a Linux machine.