Form Fields have disappeared

I had a working registration form. After going to edit it, all fields have disappeared. When I click on preview it is just a registration button. When going to add fields, the previous fields that were on the form are unavailable to use, signifying that they are already on the form. When trying to make a new form, I start by adding ‘first name’, when going on to add another field, both fields disappear. Again, I can’t pick the fields I had just added. Signifying they are already in use. But they appear nowhere. They have completely disapeared and don’t show up on the preview, backend, or front end? How do I resolve this issue?