Apple Pencil 1st gen not detected

Here is one of the better troubleshooting guides for Apple Pencil:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

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 entries for the Pencil (iPadOS13 and earlier: one each for the physical and Bluetooth connections. iPadOS14: sometimes only one entry). Finding these listed entries suggests that the Pencil electronics are good - otherwise, you have a totally dead pencil.

Check the Battery Widget for the charge status of the Pencil. Is it 100%? If not, let the Pencil 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 failing or 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. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.

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 by your retailer or at an Apple Store.

I hope this information proves to be helpful in both diagnosing and resolving the problem.

See whether the following excerpt from the Apple Support document helps you to pair the Apple Pencil again to your iPad.

If your Apple Pencil won't pair with your iPad

1. Make sure to center your Apple Pencil (2nd generation) on the magnetic connector on the right edge of the iPad. For an Apple Pencil (1st generation), remove the cap and plug your Apple Pencil into the Lightning connector on your iPad.
2. Restart your iPad, then try to pair again.
3. Go to Settings > Bluetooth and make sure that Bluetooth is turned on.
4. On the same screen, look under My Devices for your Apple Pencil. If you see it, tap . Then tap Forget this Device.
5. Connect your Apple Pencil in to your iPad and tap the Pair button when it appears after a few seconds. 
6. If you don't see the Pair button, wait for one minute while your Apple Pencil charges. Then try connecting your Apple Pencil again and wait until you see the Pair button.
7. If you still don’t see the Pair button, contact Apple Support.

Axel F.