How do I restore my photos from iCloud into Photos

It seems I made mistake somewhere but this morning i opened Photos and found all my photos had disappeared and in their place there was a message ( and I forget the correct words) but inviting to me to open another system photo library so i clicked on it and found that all my old photos from iPhotos dating from 2014 backwards, migrated there so iPhotos is now empty.The more recent photos are now in iCloud so i would like to transfer them back to Photos .Having said I also noticed that if I wish to upload any photos into Facebook the only ones I can access are now in iCloud so I am really not sure what to do because any that are now in Photos are no longer accessible. In fact Finder no longer shows Photos on the side bar whilst Pictures also seems to be displaying all my most recent photos .

Also when I clicked iCloud in finder I got the following message-" Do you want to switch the current Photos Library from “System.photoslibrary” to “Photos Library.photoslibrary”? so I am completely confused