It is possible to undo certain actions such as deletions made to your Form. While in Builder, click the "History" tab (lower-left) to view Form Activity.
In here you can review actions made by all users and Restore a deleted field if necessary. To restore the field just look for the deleted field in the history and then click on the deleted action to restore that field.
Please keep in mind the history only tracks the past 50 changes, so if there have been more changes to the form since that time the field will no longer be able to be restored through this method. If you find yourself in this situation our Support Team can run a field restore on the form for you, this will restore ALL fields deleted in the past 7 days on that form. To have our support team do this for you just file a ticket or send us an email, we will just need you to provide us with the last 4 digits of the credit card on file to verify ownership and a link to the live form.