If any form field is deleted accidentally, it can be reverted by using the form XML. Please follow the steps mentioned below to revert the deletion

Go to the forms list page
Open the form from which the deleted field has to be retrieved back.
Click on 'Export' button from the form summary page, this will download the form XML file
The list of deleted fields is available in the exported form XML underneath the <deletedFields> tag as below:

<deletedFields>
  <field udn="age"/>
  <field udn="mrns.site"/>
</deletedFields>

6. The deletion of form fields can be undone by adding the following section in the XML to be imported:

<undoDeletedFields>
  <field udn="age"/>
  <field udn="mrns.site"/>
</undoDeletedFields>

7. Move the fields from "<deletedFields>" section to "<undoDeletedFields>" which you want to undo.

8. Import updated XML file, after import deleted field is undone:

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