Apple - Old iPad not charging after no use for a year

If it won't wake after several hours of charging, try another charger and let it sit there for up to 8 hours.

You have likely let the battery self discharge to a permanently harmful level, but in most cases you can get it to trickle charge enough so that it will boot again and be of limited use depending on how long the battery was in a harmfully low voltage situation.

It may never boot again, but that depends more on how healthy the battery was when it entered storage since a year usually won't kill a new battery but will put a huge dent into it's usability.

In the end it came back after a few hours charging, has worked fine ever since.