If you have been using iCloud Photos, you need a backup a computer.
iCloud Photos is a syncing service, not a backup. Any photo you delete from Photos on your iPhone will be deleted from iCloud as well, to keep your iPhone and iCloud in sync. And when we are using iCloud Photos, the iCloud backup of the iPhone will no longer include our photos.
Perhaps you can find your most important photos in any apps where you might have shared them with someone - look in your Mail or Messages and or any social media where you may be participating. If you have a computer, check the the backup of your computer.