Try powering down. Hold the power down and wait for the screen asking if you want to shut off to come up but wait and let the Nook turn off. Wait a few seconds and power back up. This might also depend on what screen you are on. Sometimes in the settings menu the keep reading book icon isn't available. Also if you are using keep reading from the top of the Nook screen did you clear your reading history? Then there might be nothing yet to keep reading. But I suggest trying a power down and go from there.
If loving my NookTablet is a crime, I will gladly do the time :0)
