Drinking water line at door frozen ,how to fix

Move the frozen food into the refrigerator side and turn off the freezer. Leave the door open to help it thaw quicker. A blow dryer helps to speed things up. When the line thaws, turn the freezer back on and move the frozen food back over. I am curious why it froze. Was the freezer temperature set too low or has anyone been inside the door to move the water line or insulation???