Is there any way to retrieve a ticket that was accidentally deleted in WHD?
I think the only way will be to go into the database and update the job_ticket table setting 'Deleted' from 1 back to 0.
I know this works from doing it on our test server and does not appear to cause any issues.
For production you should open a support ticket to SW and ask, there could be more fields that need updated or a cleaner way.