In the form builder, under the form fields, you'll find a "History" button. Click on it to view up to 25 events from your recent form activity such as adding or deleting fields or renaming the form.
Formstack allows you to undo field deletions up to seven days after the activity. To restore a change there are two possible ways to make that happen. The first option is you can click on undo in the upper left corner of the page. This will undo the most recent change and continue to do so with each press.
The other option is you could go to the change you want to go back to and click on that item in the list and it will make the form as it was before that change.