With a USB keyboard plugged into the unit:
  1. Press the Option key (could be a windows logo, or Alt key depending on keyboard). This should bring up a menu in the upper left of the screen.
  2. Use the arrow keys to go down to "Accessories", then arrow right, and down to "Terminal" and press the Enter key. This should bring up a black terminal window.
  3. Enter the following command: sudo nano /boot/config.txt then press enter. This will open up a file. 
  4. Use your keyboard arrows to go down to the bottom of the file, and on a new line, add the following lines:
  1. Then press ctrl o then enter and then ctrl x then enter (this will save and quit out of the file)
  2. Lastly, type sudo reboot, and press enter. This will restart the display. 

When it comes back up the resolution should be back to normal.