As described, your iPad battery has failed - and will not accept charge. Li-ion batteries must be kept with some level of charge - as deep discharge will cause unrecoverable damage to the battery. iPads (and other devices with rechargeable batteries) must be periodically charged whilst being stored for extended periods.
These support pages may help with troubleshooting charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item - however, a failed battery can only be addressed by replacement.
If you continue to experience difficulties, you might be best advised to first seek assistance directly from from Apple Support. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
A better option would be to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):
Genius Bar Reservation and Apple Support Options - Apple
If battery service is required, you’ll be in the best place to explore your available options.
