When you delete photos they go to a "deleted" folder within photos. They then get purged every 30 days, so anything deleted 30 days ago will get purged. Then photos deleted 25 days ago get deleted in another 5 days, etc.
Have you checked there? Otherwise they're gone, unless you have a backup that might have those photos in them? You may also want to consider another backup service. I use the Photo Upload feature of the dropbox app to backup photos that we take just in case.