If your plan expires (it will only do this when you have actively cancelled it, otherwise it will automatically be prolonged with another month), your account will be 'set back' to our free Tiny Owl plan.
All your quizzes will continue to work just as they've always done. :)
The difference will be that you will no longer have access to certain data and features. As soon as you would chose to upgrade back, you will have full access to all your data again.
In other words: all your data, quizzes, exams, will be unaffected by up-or downgrades. Only the access to it from your account-dashboard will change.