Issue
The existing form works and can be used for new, edit and view but the form cannot be opened in the designer.
Resolution
It is possible that the form has been edited and saved in the designer but a file has become corrupted.
To get back to the previous form follow these steps.
1. Open SharePoint designer (https://www.microsoft.com/en-au/download/details.aspx?id=35491) and go to the site > All Files > Lists > ListName > Item. In this folder you will see 2 XML files.
Export a copy of NFForm.xml. This is the current form that loads when you select new item. We will use the export of the file to import back into the designer in a later step.
2. The file that is loaded when you edit and save in the designer is found using a web browser at https://<tenanturl>/site/NintexFormXml You will need to find the file that contains the matching List GUID. Delete this file.
3. Navigate back to your list and open the Nintex Forms designer. Select Responsive or Classic form based on your export from step #1. Click import from the ribbon and import the file that was exported in step #1.
Publishing this form will put the form back on the list and also allow for further editing.
Additional Information