Firstly, ensure that you have updated to the most recent version of iPadOS - at time of writing, this is iPadOS 13.7.
Here is one of the better troubleshooting guides for the Apple Pencil that may be helpful in working through common issues. This is applicable to all models of iPad and Apple Pencil:
https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/
If you have checked through potential issues using the linked troubleshooting guide, then your Apple Pencil may be displaying symptoms of a dead battery - that will not accept or hold any charge.
Assuming that you do not have any other iPad charging issues - and that the Lightning Port of your iPad is free from debris - whilst the Pencil is plugged-in to the Lightning Port of your iPad, verify that the Pencil is recognised:
Settings > General > About
Scroll to the bottom of the page - and you should find two identical entries for the Pencil (one each for the physical and Bluetooth connections). Finding both entries suggests that the Pencil electronics are good.
Check the Battery Widget for the charge status of the Pencil. Is it 100%? if not, let it charge; if it is fully charged, disconnect from the iPad.
When disconnected from iPad, does the Pencil disappear from the Battery Widget - or it’s indicated state of charge suddenly fall? If either of these are true, then the Pencil battery is likely to be dead or failing.
The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless.
If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced at an Apple Store. If within warranty and the Pencil is otherwise undamaged, Apple generally replace the Pencil without quibble.
I hope this information proves to be helpful in both diagnosing and resolving the problem.
