How do I recover lost photos on iPhone?

How did your photos get lost, Pics27Photo?

The Photos.app is only offering one safety line for photos, the Recently Deleted album. If the photos are no longer in Recently deleted, you have to fall back on your own backups to recover accidentally deleted items, see: If you're missing photos or videos in the Photos app - Apple Support

Have you checked, if the photos are just hidden or in the Hidden album, or excluded by filter? Sometimes photos are appearing to be missing, if the collections are sorted differently than expected.

In addition to léonie's suggestions, you can use the search field with the date or location—the pictures may not be where you expect, as dates may have changed. And remember, you can sort the Library by date captured or by date added. Also, your Library view now has filters, and you may not have your filter set to "All Items.”

Perhaps you have shared your pictures with friends-- then you could get them to send copies back to you.

Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look. To back up your pictures you can just plug in an external drive, select the pictures, and use Share>Save to Files. Alternatively, you can backup to a computer:

Transfer photos and videos from your iPhone or iPad to your Mac or PC

If you haven't done one of those backups, then your remaining pictures continue to be in peril.