Recovering deleted iCloud Photo Library after syncing iPad and iPhone

I recently signed into my iPad with the same Apple ID as my iPhone. After that, my iCloud Photos went missing from both devices and iCloud.com. I had Optimize Storage enabled, and I think syncing may have triggered a deletion. I’d like to know if Apple can help recover the deleted iCloud Photo Library or if there's a server-side backup still available.

[Re-Titled by Moderator]

Original Title: iCloud photos missing