Nokia Lumia 1020 - I cannot end a call because my Lumia screen remains black

  1. Press the Power (lock) key to switch the screen on, and then tap end call.
  2. If the screen remains black, restart your phone by pressing and holding the Power and Volume down keys simultaneously for 10-15 seconds.

If the problem persist, remove any kind of protective film or accessory cover to check if it prevents the touch screen working properly.