Screen is black but I can hear the notifications.

I once dropped my phone so the screen got cracks and is completely black. Now when I am sending text messages from another phone , it is receiving them and I can hear the notification sound and the light is too blinking. But when I am calling from another phone, it's showing "LINE BUSY". Is it the display which has problems ?

WHAT COULD BE THE PROBLEM ? SHOULD I BUY A LCD + DIGITIZER OR THERE IS SOME OTHER PROBLEM ???

Thanks in advance.