3D printing extruder not warming up
I really hope the extruder doesn't warm up as this would be a problem with your stepper motor :)

I'm sure you mean the hotend doesn't heat up. This could be a number of things but i would start to double check the pinout in your code vs the pins on your mainboard and measure if it gives 12v/24v (depending on your PSU) output.

There's a lot more info required to properly help you.
What board are you using?
What OS are you using? Marlin or something else?
Share the relevant code of the OS for the hotend config/pinout
potentially share pictures of your wiring