Parallax Boe-Bot servo has stopped turning

Hi! I'm new to robotics, so hopefully I'm missing something easy.

I'm using the Parallax, Inc. Robotics with the Boe-Bot book for my AP Physics 1 class. I learned how to use the servos and centered them using a screwdriver. They seemed to work fine at first - they ran code as expected, anyway - but I put them away for the day, came back during the next day's lab, and one of my servos refused to turn.

I know it gets an electrical signal because it twitches when I turn on power, but then it won't respond to any code. On top of that, I did a silly thing: When I was first troubleshooting, I tried the only thing I knew how to do, which was adjust the potentiometer. This didn't fix anything. So now the servo is un-centered and not moving; I suspect the one is fixable as soon as I fix the other, but it isn't what I wanted.

Can anyone help me get my servo moving again?

Have you triple checked the wiring?
Particularly the ground connection?

I have unplugged it and plugged it back in multiple times, yes. I also checked that the wires were connected in the right spots.

How can I tell which the ground wire is? I have a white (which Google tells me is neutral), a black (which Google tells me is hot), and a red (which Google tells me is...maybe also hot?).

(Sorry for the late reply - traveled to a wedding.)