iphoto exclamation mark

I am fairly new to mac.
I created an album in iphoto and made a slide show of my sons birthday party. It worked beautifully, until recently I created some more albums, ( all in one session).

I had copied photos that were in the photo library of the Mac. Now when I go to view these photos in iphoto they all open for a split second and then the photo goes black with a big grey circle and an exclamation mark in it ! so now I can't view any of the photos on iphoto without this happening.(even the original one of the birthday party that worked fine previously).

I spoke to applecare and they could not explain why this has happened. Hoping someone may have experienced this problem and have a quick easy solution to fix it rather than start all over again.

Message was edited by: debmarton