Start here with the iPad User Guide:
Pair and charge Apple Pencil (1st generation) with iPad - Apple Support
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 an entry for the Pencil. Finding this entry suggests that the Pencil's electronics have at least been detected by the iPad.
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.
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 purchased directly from Apple, 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 or an Apple Authorised Service Provider (AASP). Your nearest AASP may be much closer than your nearest Apple Store:
Genius Bar Reservation and Apple Support Options - Apple
If you are unable to visit an Apple Store or AASP for hands-on assistance, your options are limited. You might try seeking help directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
