Check the apps on the web app page to see if they are still available for sale. Maybe the apps got pulled for some reason.
Is your Nook the only one registered to your account? If not, check with the others using your account to make sure they did not delete the app; this has happened on more than one occasion.
Well, after more apps stopped working and more popup messages about uninstalling apps (which did not solve the problem) I finally gave up and ANOTHER factory reset, the third one since the major update fiasco. As might be expected, all of apps that previously did not work installed and worked just fine.
That's 3 resets in less than a year. After the next one Kindle might start to look pretty good.