What happens after using 10 GB with my forms data if I can't pay anymore?

What would happen with my forms data if I can't pay anymore?

Can I recover it?

And also, what happens if I only want to pay only one month?

You are free to download all your data at once, forms, submissions and uploaded files, here is a guide that will help you to export all this data at once: How-to-export-all-of-your-data-at-once

If you decide to upgrade only form one month then if you receive a lot of uploads, when you downgrade to free you will alerted about over-quota, so you will need to delete your submissions where you received uploads in order to get free space. 

To avoid loosing data you should follow the guide provided above so you will save all your received data.