Why is my Gen 1 Apple Pencil only connecting intermittently?

Involute wrote:

I have a first generation Apple Pencil and a 10.5" iPad Pro running iPadOS v15.0.2. I haven't used my pencil in a couple of years so I charged it from the iPad overnight.

Any way to get the Pencil to work?

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.

Some reports suggest that gentle warming of a fully discharged Pencil (such as placing the Pencil on a hot water radiator) prior to attempting charging, can sometimes resurrect a Pencil battery.

Hello Involute,

Welcome to Apple Support Communities! We understand you're having trouble with your Apple Pencil connecting to your iPad Pro. We're happy to assist you with this.

Let's make sure the iPad Pro is compatible with this Apple Pencil: Apple Pencil compatibility

Next, follow the steps in this article to try to resolve your issue: Connect Apple Pencil with your iPad

Best Regards.