I moved iPhoto library master files to my external drive. How do I undo this?

I thought I was freeing up space on my Macbook's hard drive by moving iPhoto library files from the Masters folder to an external drive. Now I seem to have lost some photos in iPhoto. I tried just moving the files back to the Master folder but a message tells me there's no space. How could that be if that's where they came from a few hours ago? Other forums advise never to touch that Master folder. I just want to undo this!