If it has been sitting idle for some number of years it would not be unheard of for the battery to be worn out. Lithium Ion batteries don’t like being left dead for long periods of time and is so discharged it just isn’t functional any longer.
Make sure when its charging you have it connected to the original charger, or one rated for at least 10 watts. If it is an Apple charger it will say on the brick. Otherwise wattage is Voltage x Amperage. iPads can take a good hour to power on from that level of dead if they are going to power on at all.
If that doesn’t work, then a new battery is in order.