[Roger]

Did you change the code? Specifically the number of pixels defined. (Adafruit code I think keeps a standard #define at the top of the code.) Or change a loop to turn LEDs on/off?

Check the LED strip at the point between lit and unlit. Anything broken, bent? The last 0.5 meter point is on a joint.

What is your power source? I don't run the LEDs thru the Arduino, but instead a separate 5V power source. I've had a 5 meter strip run for a couple hours on 4 x AA batteries, reducing to just the red pixels when the batteries run down.

Usually, an all-white strip may dim at the end, but not totally turn off, but I haven't had much experience in that happening. There's usually something at the end.