Dell l502x: power on, but SCREEN WON'T TURN ON, after closing/opening the screen

Hello

My laptop works well. I close the screen, the laptop goes "to sleep". All good.

But now, when I open the screen, I see the lights (LEDs, power switch etc.) come back to life - but the screen will not turn on, it stays black. Pressing the power button, keyboard, ctrl-alt-del, anything etc. will not make the screen come back on.

I tried to use the Dell utility on the webpage to update bios, drivers etc. but it didn't seem to do anything.

What can I do?

Thanks in advance!