For iPad models with a Home button, do a simple hard reset of your iPad by holding down both the Home and sleep/wake/power button simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
OR
For much newer design iPad models, without a Home button, try the new simple hard reset procedure for the new 2018/2020/2021 iPad Pros, 2020 iPad Air 4 and 2021 iPad Mini 6 models.
1. Press and release the volume button closest to the top Power button
2. Then immediately press and release the volume button farthest from the top Power button.
3. Then, press and hold the single sleep/wake/power button You will see the Slide to Power off button, but keep on holding down the power button until you see the Apple logo, then let it go.
Once you have performed all the steps, wait for a few seconds and your 2018 iPad Pro will boot up completely to the iPad Pro’s Lock screen.
If no success,
Try other known good, compatible power/charging cables and/or other known good 18 or 20W power/charging adapter blocks to test for current cable and/or power/charging block failures.
Power/charge cables and/or power/charge adapter blocks can AND DO fail at any time for any reason.
If no spare power/charging cables or power/charging adapter blocks or NO joy on getting this iPad charging, you can purchase new cables and power/charging blocks from your nearest, “local” Apple Store location or from Apple's own Website.
OR
Purchase new, third party Apple MFi ( Made for iOS devices ) certified power/charging cable and adapter block, from online retail, mobile device sellers, to test with, in order to determine that your existing power/charging cables and adapter block is bad/faulty.
OR, if no success with brand new power/charging cables and/or power/charging adapter blocks,
You may need to use the “Support” link, in the top black, menu bar, at the top far upper right hand corner of every Apple Support Community Webpage, and make an appointment for your nearest, local Apple Store and bring your iPad and charging cable and adapter block, to the appointment, for Apple employees to test/diagnose and options presented to you.
Best of Luck to You!