Something like this happened to me recently, although I assume it's because I'm doing too much modding on mine. My device was fully charged; power button and reset button did not work; I had to open it and disconnect the battery [which is what I assumed the reset button would do but apparently there is a difference].
You should explore all other options (not sure which ones your are, your battery indicator looks fully charged as well) before actually opening it though; doing so will destroy the water seal and it won't be quite like before (the device is glued shut and on mine the glue got "lumpy" when opening it). And you risk breakage of the display which just happened one thread over
https://www.mobileread.com/forums/sho...d.php?t=264129 
or you could lose touchscreen functionality if anything happens to the bezel.
You could let it run out of battery naturally but it might take years. Depending on whether it's stuck in an endless loop that's good at sucking the battery dry, or in a standby-esque ultra-low-power-consumption mode...
Is the light still on?