If you do not renew your account before its expiry date, you will automatically be downgraded to a free plan. So make sure you make payments before you next billing date.
What happens to my collected data?
Your already collected surveys and response will remain yours and are stored in your account even when your survey is reset back to a free account.
However, you will be limited to a set of features because a free plan has restricted access to paid features such as exporting of data and more.