iPhoto exclamation mark

Having moved several thousand photos from my old p.c. to my iMac (10.6) I find MANY photos visible as thumbnails but when I try to bring them to full screen or to print them I get a full screen (grey) exclamation mark. The photo in question is 173K so should be large enough to show full screen. Please explain the exclamation mark and tell me how to access the photo so that I can print it/edit it.
scozzer
So.Calt

Thanks for the response DJ,
While the 'Command/Option/Boot iPhoto' did make some significant changes in the photos I can now see, the photo that I particularly wanted still converts to an exclamation mark when I bring to full screen.
"Revert to original" is greyed out both in the thumbnail and full photo.
Scozz

Thanks for your attention and patience, DJ.

Your suggestion to export to desktop results in pop-up that says:

Unable to create : User/m****/desktop/**
This issue is annoying but so much so that you should spend anymore time on it. I did find that some of the exclamation mark photos can be emailed and the picture there is bigger. I suppose I could take a screen shot of the one I want and enlarge the email.
What does the Exclamation Mark icon mean ??