Laptop screen stays black after my toddler turned '_'?

After my little daughter typed on the screen while it was shutting down, my laptop started up - but the screen remained black.

I have already tried common tips such as Fn + F5 / F7 or adjusting the screen brightness or attaching a second screen, unfortunately without success.

Do you have any helpful tips?

It's Fn + F8 on my DELL Latitude