This recently happened with my my mom's PRS-650. Froze, couldn't do a reset, PC did not recognize the reader when plugged in.
Sony had me do a reset with the reader plugged into the PC. This seemed to work and then I charged it fully on the PC.
I played around with it to make sure it was working...it seemed to work fine except for the book she was reading...so I removed it. (In addition to a possible problem book, she also used a phone charger instead of the computer cable for charging...maybe just another coincidence).
However a few days later, I turned the reader on one last time before sending it back to her and got "exceptional internal memory" error and automatically formatted itself.
Fortunately, this happened just within the 90 day window, so I got a refurbished replacement sent to me.
I've already had to reset my reader...and to be honest am worried these might not be as reliable as the other readers I have owned which lasted for years.
But I do love the device and so have my fingers crossed that this is just all coincidence.