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The most important thing is to check the "Recently Deleted" folder in the Photos app right away - photos stay there for 30 days before they're permanently gone. If you've already deleted them from there, you can try to recover them from an iCloud or iTunes backup if you made one before deleting them - just restore your device from that backup, but be careful because it'll overwrite everything you've added since then. If you don't have a backup, unfortunately there's not much you can do on your own; there are data recovery software options for Mac/PC but they don't guarantee anything and require you to connect your iPhone to a computer.
If the photos have disappeared from the Recently Deleted folder, your iCloud backup is your lifeline - you can restore your entire device from a previous backup or use iCloud.com to check if they're still there in cloud form. I accidentally deleted a bunch of shots and found out they were still in the backup, so before you mess around with full restores it's worth checking there first.
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