I am currently trying to connect a LCD from Adafruit to an Arduino Uno. When I connect the 10K potentialmeter to the 5V the LCD backlight turns off. I tried with several 10K ones but all had the same result. Help???
You have something connected to ground that should not be. Possibly you have confused the potentiometer pin for the wiper with the pin for the ground connection.
For a start, connecting the potentiometer to 5 V is actually a bad blunder which has been propagated from one ill-conceived "tutorial" to another for many years now. It absolutely should not be connected to 5 V in any case. Not connecting it makes setting contrast twice as easy (even easier of you use a 1k potentiometer instead) and saves an unnecessary current drain.