There are three current components to Apple's online photos.
- iCloud Photo Library (Beta)
- My Photo Stream
- iCloud Photo Sharing
I think from your wording you are referring to the smaller parts and not the entire Photo Library in the cloud. The photo stream FAQ is quite good:
It mentions that you have a 30 day rolling window to save files that get stored in the stream. If you lost photos before that time - you can get help from this article:
Basically, you can use another iOS device or a PC/Mac to access the stream and see if the corruption is in the stream or on the device.
If you have backed up the photos, you can restore that iOS backup (or PC/Mac Backup) and get things back based on how recently your backups run.