How to retrieve deleted iCloud Photos back?

Then ask Apple Support for help. Explain, that this is an emergency and that you did misunderstand how iCloud is working and need help. If it has not been too long ago, Apple Support might be able to recover your iCloud Photos from their own backups.

It is a common misunderstanding. I m sorry you got trapped by it. We are just users here and cannot help with accessing Apple's servers.

Apple's iCloud is a syncing service, not a separate external storage. We should use iCloud, when we want to keep the Photos Libraries identical across all our devices, but not as a separate external storage. When we delete a photo from one of our own devices, iCloud Photos will remove it also from iCloud and all other synced devices. We are having essential one version of our Photos Library in iCloud, and it is mirrored on our devices. When we modify it on one device, the modification will be synced to iCloud and to all other devices.

See: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community

Your Recently Deleted folder is the only safety line offered by Photos to get deleted photos back.

Once you emptied it, you have to fall back on your own backups. Have you saved your photo on a computer, for example?

You're welcome. I hope, Apple Support will be able to help you.