Form disappears showing only fillable fields

Your fields have no border color and no fill color: select all and give them colors.

The document has only form fields, nothing else.

What have you done after adding the form fields?

Not sure what you did in the form but if you are talking about checkboxes, select 'prepare form' tool and select all of checkbox fields, right click on one of them, select properties and try to change their border color (you can select black again).