Try in Settings app forgetting the device in the Bluetooth Settings,then try re-pairing.
If still no success, try a 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, once your iPad Pro comes back, In Bluetooth Settings, again, forget the Apple Pencil (Forget this device and re-pair).
If still no joy,
Try letting it charge on the iPad or the supplied adapter for a couple of hours without using it.
Then, try re-pairing it after a couple of hours.
If STILL no joy, the Li-ioj battery has been completely depleted and the Apple Pencil is dead.
You will have to purchase a new one.
With new Apple Pencil make sure it is always charged, even if it is not being used for prolonged periods of time.
This is yet one of many reasons I do not like the Apple Pencil. It is always in an “always on” state and the Li-ion battery just constantly drains.
This is why I use another third party smart Bluetooth stylus that works very will with my iPad Pro with no issues like the many that have been ocuring with many Apple Pencil users.