Sorry for your loss. Check the "Recently Deleted" album at first.
Have you done a iTunes or iCloud backup in advance? If so, you can easily restore those lost photos from backup files. Log in iCloud.com and check for them. Try the followings steps to restore them. iTunes: Connect to iTunes > "File" > "Devices" > "Restore from backup.." iCloud: Reset your iPhone first by "Settings" > "General" > "Reset" > "Erase All the Content and Settings". After that, choose "Restore from iCloud backup" when iPhone reboots.
Note that both methods will erase your current data stored on iPhone. If you think it's worthy, give it a try.
In the case that you have no backup files available, I'm afraid you need to try some software for help. Please stop using your iPhone lest the data be overwritten. I've tried iPhone Data Recovery (Found on iFoneTips) for my playful son deleted some photos by accidental, and that's how I got my lost photos back on iPhone 6s.