Start with the easy stuff. Is there water at the refer? Remove the line from the back and check for water flow. If equipped, is the filter plugged (water flows without filter in sort of thing)? Is there debris in the line to the refer blocking the valve? Does the water valve solenoid "click" and flow water if you remove the internal water line that goes to the frige interior? Is the water line frozen in the door (turn off frige for a day and check water flow).
Usually answered in minutes!