Black screen

[Made in flanders]

My girlfriend has a lumia 620. All of a sudden today the screen went black. If I call her phone he still rings, and if she touches one of the three buttons down below, he still vibrates, so it seems like the problem is just the screen, but the phone self still works.
My phone is a Blackberry, and I'm not really familiar with what I can do now with a WP.(With BB it's normally a battery pull, but here this isn't possible ).
Does anyone know what the problem is, and what I can do?
I suggested my girlfriend to buy a WP, and I'm sure she'll be pretty pissed if the phone wouldn't work anymore.
​Thank you

 

[N_LaRUE]

Battery pull on a WP, known as a soft reset -> When on hold volume down and power button until phone vibrates. No files will be deleted. Date and time may need to be reset.

Try that and let us know.

 

[Made in flanders]

Thank you for your help.
I just tried to do that, but the phone didn't vibrate.
The phone stopped responding to anything actually, it doesn't vibrate anymore when you press some buttons, and when I call it, my phone keeps ringing, but her phone doesn't ring anymore, so I'm not sure if it is turned off or not.
I tried pressing a lot of buttons before, so maybe I did something.

 

[Made in flanders]

Never mind, I tried it again (pressed a long time) and all of a sudden it vibrated. He did reboot and the screen is back on again.
​Thank you very much!