I just want to rant here...for older ipads especially....i got this issue before but now after i go for biz travel for 1 week and forgot to charge...i left 2 of my ipads and they completely discharged...

now very hard to charge back again...apply ipad really sucks on this point.

on one ipad (newer one)...totally blank screen....

on ipad mini (older version)...battery icon with one red line blinking non stop...

really damn frustrating ....

i felt the same, some of my ipad put in store room long time, when need to used need to charge it, really take long time to full charge

usually, before sleep i put in charge, when wake up it almost completed
butif adhoc, of course kinda headache

Just plug in and leave it overnight and it will charge. My organization still have quite a number of ipad air 2, sometimes will fully discharge and left for few days, same issue.

Perhaps you can consider to change battery at Apple Store

For the blank screen on your newer iPad, have you tried a hard reset? Sometimes that can kick it back into gear.

You can’t hard reset a flat battery though…