Photos can't find original files
Since switching to Photos for Mac, when I double click on a thumbnail to enlarge, a large percentage of my photos have a message that the original file can't be located, and offers me a chance to find the missing file.
In iPhoto, where most of my photos migrated from, I would simply store photos within the program. As far as I knew, that's where the original file was located.
What's happened to my photos?
iMac, OS X Yosemite (10.10)