Ipad won't boot up. Apple logo flashing/bootloop. All help is welcome!

Hi guys!

The ipad pro 12.9”(1st gen) It wasn’t used for over a year. When booting it up after that period, the apple logo appears and disappears a couple seconds after it. This goes on and on. But it will only work when the ipad is charging.

What I tried:

• Charged for 45min on charger
• Hard reset, with no luck.
• Putting the ipad in recovery mode, the itunes logo appears, stays on, but when I want to connect it to the pc it instantly shuts of the display. Pc can’t charge the ipad.

Might this be a battery problem? That the battery can’t hold it’s charge when charging. So the battery is ‘empty’. And there is not enough power to boot it up when connected to the charger?

Thanks in advance!

Nick