Q: HELP: Recover Lost Files!! Photos/Videos still in iPhoto but 'lost'.

Recently my MacBook kept saying that my disk is full. Many people had recommended me to download and use OmniDiskSweeper. With that, I'm not sure if it completely wiped out everything that I have or not. I am still able to view photos and watch videos normally in iPhoto (that's where they were initially at) however, when I try to export the photos and videos to elsewhere, I'll get an error message saying that the file is 'lost' or just not there to export. If the files are not there, I should be able to view them in iPhotos right??? I tried looking everywhere to see if there could be a backup somewhere but I can't find it nor am I able to export it at all just through iPhotos. Can someone please help me. I would greatly appreciate it!