In one word, this is happened to me "Cookies". I maintain several Gmail accounts sharing documents and calendars, ect.. It used to be, you could log into your Gmail and my Google Docs account would know who you were logged in as. Something has changed just recently. Now when you click documents from your Gmail account "It does NOT know what account you’re in because they are using separate cookies now.
1. In the Google Docs Window you need to hit the logout in the upper right.
Make sure it’s the Google Docs Window and not your Gmail.
2. Then log in using the Gmail account you wish to view.
All the while I thought I was logged in to my Google Docs account. I was in-fact not. I was viewing the documents one of my client's and thought I lost a whole lot of files. “Shew!!!”