Plug the Sony Reader into its power supply if the eBook page appears to be frozen on the screen. The device may have run out of power and the lack of power is stopping your button presses from changing the page. A lack of power can even prevent you from turning the Reader device off.
Slide the power switch on the Reader after about five minutes to see if the issue is corrected. If it isn't, perform a soft reset by pressing the "Reset" button. Then try the power switch again and let the Reader start up.
Plug the Sony Reader into its power supply for a full hour if you continue to have trouble with the eBooks freezing. Take a look at the indicator light and charge the device until the light is no longer red. A full charge takes four hours with a computer via USB, and two hours via an AC adapter. Make sure the laptop or computer that you are using to charge the Sony Reader doesn't go to sleep because charging will stop and the Reader's battery may even start to become depleted again.