I just installed a brand new VEX Power Expander into my NbN robot.
It worked great for about 10 seconds, and now all it does is twitch and make angry motor noises. Is it a programming issue? or is something broken?
I have checked all the wires, and they are fine.
You should probably elaborate on what “It” is. Is “It” the entire robot? or just the motors connected to the power expander? or what? Is the cortex still working okay? Are any sections of the robot performing okay? Are you running this with a joystick or entirely by a program? Are all your batteries charged up and connected?
I’m going to take a wild guess that it’s a mechanical problem, that something is jammed or that you have two motors fighting each other (one is driving in one direction and the other is going in the opposite direction perhaps because of a programming error or motor wires switched around). But that’s just a guess. We need more description about what the robot is doing and what’s going wrong. And if you can supply a photo, that would be even better. 
Check the VEXnet connection and your keys as well.