There are several things you can do. First make sure all windows are closed (tap the settings icon in the top right corner) and you will be taken to your set home page. You can also clear all cache and cookies under settings.
Or you can shutdown and power up the Nook it's self which clears all you extra open windows which is probably your problem. Hold down the power button until the screen blurs - then continue to hold it down until it shuts off. Wait a few seconds then hold down the power button until it starts up.
This exact thing has happened to me a few times over the last month month I've had my nook color - only solution I've found (that works) is to power the device down completely via on/off button on top left side Must be a browser quirk.