Go to Settings > Your Name, Apple ID, iCloud, iTunes & App Store
Tap iCloud
Are Photos turned on (green)?
If yes and you delete photos, they delete everywhere, including in the cloud.
When you delete photos however, they go to a folder called Recently Deleted, which they hang out on for up to 30 days so you can restore photos you may not want to permanently delete. Did you check the Recently Deleted album?
If Photos are not turned on in iCloud, and you have a backup made from before you deleted the photos, you can restore your phone from that backup, but anything you had on the phone from the date the backup was made will be gone.