What can you do?
Check to make sure you are not using a Teacher account, and that you are connected to the internet. If so, you can contact Support and send an email with as much description of the problem as possible, and the developer will get back to you as soon as possible.
Why does this happen?
TeacherEase changes their website quite often. The way NAG Notifier is designed makes it highly sensitive to whenever TeacherEase.com is changed. If a formal arrangement with TeacherEase can be worked out, this will be fixed once and for all, but until then, you will have to wait.