What can i do to continue using free form? They stop working.

This is because you have already reached your upload space limit. For "Free" plan, you are allowed to have 100 MB of upload space.

To continue using the forms, you need to either upgrade to a higher subscription plan OR delete submissions with file uploads on them. Please note that deleting submissions will permanently delete the submission data in our servers, it is irreversible. 

I would strongly suggest that you take advantage of our integrations related to 3rd party file sharing and storage services (e.g. Dropbox, Google Drive, Box, etc...). That will create a copy of your submission data and upload the file in those account, giving you a real time backup, however, please note that this will apply to new submissions once the integration has been done.

I hope this helps.