there is no ‘automatic’ resend method and you will basically have to program this into your form and using the ‘new version’ and ‘map versions’ functions, deploy a new version of the form, transfer the existing instances into this new form so you can then resend.
The form will probably be in the ‘next stage’ but you can look at adding in a method so you could reload the form in this stage and then resend the notification but not then changing the stage of the form when you submit. Something like a ‘check’ box you need to check on the form and then if your workflow path conditions are reviewed for the next stage the form should have been sent to so it will only go to the next stage if this checkbox is NOT selected, then the form can be submitted, a notification sent but it does not go into the next stage.
Remember of course that you will need to ensure that this ‘check’ box is unselected as the form is then accessed by the correct users so it will then route to the next stage as expected. Publish the form as a new version and ‘map’ the old instance into this new form, so you can then re run the form
I would say thought that it may be best initially (assuming that the user does not have a ‘deleted email’ folder he/she can access to find what they deleted) to talk to your mail admin as they may have a log of the emails sent and can ‘resend’ from there ?
Also though, and dependent on how many ‘users’ you have in your account, if you use the ‘attention is needed’ function in the email notification, your users can always access the form from the notifications dashboard as well if they delete emails that they shouldn’t