Problems opening .psd file

I was working on an icon till late at night and I saved three versions of it as a .gif, .jpg and .psd. Now I want to do some more work on it and the .psd file will not open. I get a message saying that the request to open could not be completed because it is not a valid Photoshop document. Has anyone else had this experience. I'm using Photoshop CS (PS8).