Venue 8 (android won't boot)

I have a Dell Venue 8 (android) that i had left sitting for months. It was working fine previously.

I decided to charge it and start using it again. When I charge the battery icon shows 100%.

If I unplug the charging cable it will not turn on. If I charge and turn on with the charging cable still connected it will boot. As soon as I unplug the charging cable it goes dead.