Photos disappear/become invisible

Wondering if anyone could help me, never had this problem before...

I've had photos on my Macbook Pro for a while now, and I've just gone to view them and loads have disappeared. I have a lot of folders to sort photos out, all of the folders are there however the content in most of them has gone. Only photos that I've very recently uploaded are still there. I also noticed that when inserting a photo from iPhoto into the folder in Finder, it immediately vanishes even though it says the file is there and makes the noise.

Please help!

Thanks.