Leave it charging for a few hours. If it still boot loops then press and hold 2nd and del (they're the top full sized buttons on the keypad) then press the reset button on the back of the calculator with a pencil for at least two seconds. You should see a message that says "send OS now". Do not press clear.
Leave it charging on this screen for another few hours. Afterwards, press and hold the reset button on the back of the calculator for at least two seconds again (but don't hold 2nd and del this time). This might cause the calculator to boot normally.
If you get the same "send OS now" screen then press the clear button (rightmost column on the keypad) and proceed with an OS reinstall: [guide|111572]
If it still boot loops then you likely got a defective unit. I'd return it for a new one.
If returning isn't an option, you can try buying a new battery and replace it. [guide|63973]