I haven't turned on my tablet for months, and now it refuses to boot past the logo or turn off. I've waited for 2 and a half hours at this point, and nothing has happened. I even tried the reset button, and it went back to doing nothing.
There's usually a corner which has a small slot. If you can see that and if you have an old credit card, put that into the slot and twist. Run the card all the way round the case and definitely don't use a knife because the metal will upset things.
It's the big silvery bit on the right and personally, I wouldn't try removing or disconnecting it myself. Those tiny wires are extremely vulnerable and if one or both break away from the connectors to the battery it's a difficult job to reconnect them. Before you go any further, see if you can see and remove your personal data, photos, contacts etc. by connecting it to a PC using a USB cable.