Oops I never came back to update this particular thread.
I managed to open it up and remove the battery, then I did what some troubleshooting videos have suggested: to charge the device without the battery then putting the battery back in, and now all problems are gone. It basically is business as usual, with battery life back to normal.
I'm suspecting the battery was dislodged as a result of my throwing it around at the beach and bed and other places.
