My husbands NT is not opening his books. (Un-rooted device, all eBooks are side loaded, not purchased content.) He had about 20 pages left a particular book and when he tried to open it this morning he got the error message "Activity reader (in application reader) is not responding". No other book will open now, either. We tried powering off and on again several times and I even deregistered and registered again. That sort of worked, until he tried to skip ahead to where he left off in the book, then it froze and gave the error message again. What else can I try?