Lost photos at apple store

Usually the email confirmation of an appointment at the Apple Store is stating that we should make a backup before bringing a device into the store to be serviced. Did you make such a backup? Then restore your iPhone from the backup you made, see: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

There is one complication: If you have been using iCloud Photos, when you made the backup, the backup will not include your photos, as they are in iCloud. In that case you have to enable iCloud Photos again, so your photos can be come back from iCloud. That should work, if you did not erase your photos from iCloud since your iPhone has been serviced. See the paragraph "Make sure iCloud is turned on" in this document: If you're missing photos or videos in the Photos app - Apple Support

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support

- "To protect your information and save time, follow these steps before you get service for your device."

Do you have iCloud turned on?

Sign in to iCloud on all your devices - Apple Support