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If the iPad has been stored in a discharged condition for a significant period of time, the battery may have suffered irreversible damage through deep-discharge.

An iPad requires a Power Adapter to be rated at 12W or greater to reliably charge. Many iPhone Power Adapters are only rated at 5W - and are inadequate.

If the Power Adapter is powerful enough, connect to the iPad and allow the iPad to charge, undisturbed, for at least 24 hours. If you are lucky, you might be able to recover the battery sufficiently to power-on. Otherwise, the battery will need to be replaced.

You make no mention of your iPad model.Depending upon the age of the iPad, a battery replacement may not be worthwhile.

So, the battery is already over four years old - but the iPad is still in support.

If the iPad is otherwise undamaged and fully functional (the latter being impossible to demonstrate of the iPad will not power-on), Apple will usually offer an “exchange” for a reconditioned iPad of the same model and specification for $99 USD plus tax (or local currency equivalent). Otherwise, the out-of-warranty charge will be levied - this typically being c.60% of the “as new” price of your iPad.

iPad Repair - Official Apple Support