IPAD PRO ( 3rd Generation) CHARGING ISSUE

Given the period of time that the iPad was set aside unused and not recharged, it is very likely that the iPad's Li-ion battery has been un-recoverably damaged through deep discharge. If the iPad will not successfully restart after being connected to its Power Adapter - and allowed to charge undisturbed for 24 hours - the battery is dead.

You essentially have two options - these being to (a) replace the iPad's battery, or (b) replace the iPad.

For practical purposes, for battery service of qualifying devices, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for battery replacement. When exchanged, the replacement iPad is yours to keep.

You can verify both eligibility and cost for your country/region here:

iPad Repair & Service - Apple Support

Before committing to a battery replacement, you might be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider where your iPad can be assessed by Apple's technicians - and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

There are a number of possibilities, including debris in the charging port, a bad cable or charger, or a failed battery (though if the iPad indeed has never been used, a failed battery is less likely though not impossible after sitting for years). You'll find some suggestions in this Apple support article that will provide at least a starting point for troubleshooting:

If your iPad won't charge - Apple Support

If none of that works, then you'll probably need to take the iPad to an Apple Store or third-party service provider for diagnosis and an estimate for repair.

Regards.