Images imported from a digital camera were viewable and editable at first but later disappeared! They cannot be re-imported from either the camera or the iCloud as iPhoto indicates that they already exist, but cannot locate them. Please help!

On holiday I have been uploading my photos to iphoto on my macbook pro every few days, the latest batch appeared and I was able to edit them, flag, merge events etc. However, later that day when I went back to the images they could not be found in my library. They had been removed from 'latest import', however were viewable on the icloud photo stream. I have attempted to re-import the images from both the digital camera and my photostream but iphoto indicates that the images had already been imported and would not import them again. I was able to locate the master images using finder, but Iphoto tells me that these files are unreadable (they are .jpg) when I try to re-add them this way. I have also tried to 'unhide' all images in case I had accidentally hidden them but this didn't work either.

Any other suggestions?