All in favor that's its a dead LCD?
It's more likely to be an 'extended temperature' display in which case it needs a negative voltage on pin 3.
To check this out you will need another power supply, a 9-volt battery will do. You will need your original power supply connected to pins 1 and 2 of the LCD as before.
Connect the POSITIVE terminal of the battery to GND and the negative terminal to one end of your potentiometer.
Connect the other end of the potentiometer to GND and connect the wiper to pin 3.
Now adjust the potentiometer and see if you get a row of boxes.
Don