I enabled iCloud photo library on my iPad and all pictures from photo stream are gone now. How can I get them back?

I wanted to upload all pictures from iPad 3rd gen to my new iPhone 6 both running on iOS 8.3. When I enabled iCloud photo library on iPad, last 1000 pictures disappeared from it completely instead of being uploaded to iCloud photo library. I understand that iCloud photo library completely replaced photo stream, but I don't understand why my photos got deleted instead of being transferred to the cloud.

How can I get my photos back?