Your Pencil battery may be exhibiting symptoms of early or unexpected failure.
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 (or at least tested) by your retailer or at an Apple Store.
Some reports suggest that gentle warming of the Pencil (such as placing the pencil on a hot water radiator) prior to attempting charging can sometimes resurrect a Pencil battery. Whilst nobody should advocate external heating of any Li-ion battery (this being potentially very dangerous), gentle external heating of a tiny pencil battery is unlikely to lead to catastrophic failure.
