Why is my iPad not charging

This could be a whole lot of things, I’ll run through the obvious first.

Make sure your USB cable, your power brick and the wall outlet are good (do they charge other devices? Try a different charger if you have one. If the battery was dead, these can take a bit of charging before they will come back on again. I’ve seen especially dead iPads on slower chargers take 15+ minutes to even get an indication that the unit is charging, let alone powers up.

Connect it to a computer, see if it is recognized. Even if it just prompts that it is in recovery mode, or that the passcode needs to be entered, at least we know it’s powering on and communicating via USB.

Does the lightning cable sit flush with the bottom of your iPad? If it sits askew, or sticks out a bit, there back be some debris stuck in the port preventing the charger from making proper contact.

Beyond that, you would be talking about replacing components. I would rather cover the basics first.