Photo and Video files disappeared

See the article If your iCloud Photos aren't syncing to your device - Apple Support

If your iCloud storage is full or almost full

If you exceed your iCloud storage limit, photos and videos won't sync to iCloud and your library won’t stay up to date across your devices. To continue using iCloud Photos, you can upgrade your storage plan or reduce the amount of storage that you're using.

Note : it's not necessary the photos are still on iCloud server , they were not saved as iCloud storage was short , if you decided to upgrade the iCloud storage a sufficient space must be there , you need to check the status of iCloud storage Check your iCloud storage on any device - Apple Support

once iCloud storage is sufficient , if you upload new photos / videos they will sync with other devices ( iPhone / iPad ) Access and view iCloud Photos on your iPhone, iPad, or Mac - Apple Support in which photo library is set ( turned as on ) Set up and use iCloud Photos - Apple Support

Make sure photos are not hidden ,Hide photos on your iPhone, iPad or Mac - Apple Support 

Hide photos from view in Photos on Mac - Apple Support 

"Photo and Video files disappeared: A lot of my photos and videos from the past month or two have disappeared from my phone AND my iCloud and I don’t understand why. It was all backed up yet it’s still gone and I can’t find it anywhere! It’s not in my recently deleted or hidden folder and I didn’t delete them or hide them in the first place. I don’t know what else to do or how and if I’ll get them back. Did this happen to anyone else?"

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Troubleshooting Deleted Photos:

Loss of data is something you would have agreed to upon opening an Apple Account. So, if the following does not work, then they are gone. That's just the way it is :(

Four Tactics to Try:

A. Start here: If you're Missing Photos or Videos in the Photos app - Apple Support

B. Check iCloud, Directly:

For Photos: Go here, and see if they reside in iCloud: https://www.icloud.com/photos

C. Restore your iPhone from a Backup:

Have any backups of this iPhone prior to this occurrence? If so, restore your iPhone from one that has these photos (the most recent backup, ideally). If not, then take this as a learning experience, knowing to always back up your device for recovery purposes. Important: Back up you iPhone prior to performing this restoration, should anything go wrong from here on out.

D. Where Else to Check:

1. PC
2. PDA
3. eMail
4. Mac Device
5. Cloud Storage
6. Android Phone
7. USB Flash Drive
8. External Hard drive
9. Secure Digital Card
10. Compact Flash Card

"Photo and Video files disappeared: A lot of my photos and videos from the past month or two have disappeared from my phone AND my iCloud and I don’t understand why. It was all backed up yet it’s still gone and I can’t find it anywhere! It’s not in my recently deleted or hidden folder and I didn’t delete them or hide them in the first place. I don’t know what else to do or how and if I’ll get them back. Did this happen to anyone else?"

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Troubleshooting Deleted Photos:

One unique method...

Download All of iCloud:

One thought would be to download all of iCloud, and then search through it in Finder on a Mac. You would download it to an external drive, and then open the drive in Finder. Search through it by typing a filetype (i.e. jpg) in the search bar on the top-right corner of the Finder window. Use my user tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip