What should I do when my Apple Pencil stops working completely?

Assuming that you have an Apple Pencil that is compatible with your iPad, this support page may be helpful with pairing and use of the Pencil with your iPad:

Connect Apple Pencil with your iPad - Apple Support

If you continue to experience difficulties, here is one of the better troubleshooting guides:

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

Be aware that if your Pencil has been set-aside for an extended period, unused and uncharged, the Pencil battery may now be irretrievably damaged through deep discharge. In common with all Li-ion batteries, they must always have some level of charge.

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.

Some reports suggest that gentle warming of the Pencil (such as a hot water radiator) prior to attempting charging of the Pencil can resurrect a Pencil battery. Whilst I do not advocate external heating of any Li-ion battery (this being potentially very dangerous), gentle external warming of a tiny battery is unlikely to lead to catastrophic failure. 

Your Pencil battery may have failed and your Pencil is dead.

Sorry.

You cannot store and/or NOT use Pencils for any prolonged period of time and NOT keep them charged up to some level.

If your Pencil/s sat around unused or unopened and not kept charged up for more than a few weeks, or so, OR

EVEN LONGER, then the battery in your Pencil may have failed and is dead and you will have to purchase another

brand new Pencil or get, if still under warranty, a free replacement Pencil from Apple.

Sorry.

And it looks like some things about the Pencil have NOT changed in the new version 2 model, either.

FYI and something Apple will never, EVER divulge to it potential iPad Pro/Pencil customers/users unless the potential customer/user is smart enough to ask about,

Due to the “always on and active/standby” nature of the Pencil and, also, due to its very tiny rechargeable Li-

ion battery ( smaller than the eraser head on a REAL wooden pencil ), the Pencil needs to be constantly charged up to some significant charge level ALL THE TIME, even when not using the Pencil for prolonged periods of time.

Apple/Apple Stores or Apple employees will never, ever, mention/divulge this long term issue/flaw of the Pencil.

The Pencil needs to be kept charged to a minimum of 5-10%, OR GREATER, all the time.

If the Pencil battery is allowed to drain down to 0% and allowed to stay in that flat condition for more than a few weeks, or so, OR EVEN LONGER, that very tiny little Li-ion battery is too small to keep at a 0% state for a long period of time and it will fail and the Pencil will be dead and you may have to “pop” for another $99/$129 Pencil, once again!

Apple only has a one-year warranty on any Apple accessory items.

You need to keep your Pencil charged all the time, even if it is only to 10%-15% charge. It must keep some charge level in that tiny Lithium-ion battery at all times!

One last ditch thing you can try is to plug the Pencil's Lightning connector into the iPad.

Then, try a simple hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

OR

Try the new hard reset procedure for the 2018/2020 iPad Pro/iPad Air 4 models without a Home button.

1. Press and release the volume UP button.

2. Then immediately press and release the volume DOWN button.

3. Then, press and hold the Power button at the top. 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 iPad Pro will boot up completely to the iPad Pro’s Lock screen.

See if the Pencil starts charging then, when it reaches a sufficient charge, like over 15%, or so, or more, disconnect the Pencil and reconnect it to see if it Bluetooth pairs.

If it does pair, try using it.

Someone else posted another possible idea/solution that you can try before completely giving up on your Pencil.

Try heating up your Pencil using a hair dryer on its highest setting for 5-15 minutes, or so.

Do not get the hair dryer heat too close to the Pencil.

Or try leaving it near an active home heating register, where you live, for 10-15 minutes, or so.

See if this will revive the Pencil battery, or not.

If still no joy, your Pencil is definitely dead.

Sorry and Best of Luck to You!