This issue is a PITA to get the iPad started up and working again.
Make sure you are using a 2A apple wall charger and a good charging cable.
Keep it plugged into the iPad.
Try doing what is explained in this link:
https://daniellehatfield.com/2014/05/how-to-save-your-ipad-from-the-loop-of-doom-a-true-story-of-patience-and-curse-words-by-danielle-hatfield/
On a side note, I don't know why apple didn't just implement a charging screen like android devices do when powered off. This would save a lot of hassle with worn out batteries as it forces some users to get their batteries replaced because of this flat battery boot loop issue.