First make sure the contrast is properly set. Look at the extremes of your potentiometer. When turning black you should recognize the pixels look closely. I would first set the contrast to the position where the pixels just start to appear. Once you see some recognizably text you can always turn it back to your liking.
Also make sure your Arduino is running by adding a blink+delay in your loop().
Also check the hints given in this topic and the other topics listed at the end of your topic.