Laptop screen first black then white?

I connected a second monitor to my laptop. It went very well but after 3 weeks my laptop screen was only black but the taskbar at the bottom is still there but only the home screen is or was black now it is white. How do I fix that?

Press Alt + F4 times

It is best to use the reboot system, Google your PC (serial number is usually below) then google the key combination of your laptop to reboot, there you can go back to an earlier memory status.

For me it was always like that, only that helped.

There are no tabs open so it doesn't help. Tried already