Photoshop CS3 Cannot Crop image

Hello everybody,

I have photoshop CS3 installed on my MAC at work and have been working on a particular .PSD for the past week. all was fine until this morning when i tried to crop the image and received the message "could not complete your request because it is not a valid photoshop document"

I've read about people being unable to open files but i can close/save/open to my hearts content, even more strange is that no other editing tools seem to be affected. (as yet)

please help!

thanks,

Sam