Fortissimo, was your battery low? Here is some info on power management from the 950 User Guide. I assume the 650 is similar.
Power management operates as follows:
- After 10 minutes of inactivity, the Reader enters the sleep mode automatically and battery consumption is minimized.
- After three* days of inactivity, the Reader automatically shuts down and stops consuming battery power. * If battery power is at a low level, the Reader will shut down after 60 minutes of inactivity.