Is the file you are looking for missing completely, or are you trying to recover a previous version of it. If the latter, we automatically back up all of your previous versions to provide you with revision history, so you never lose track of what you've worked on and when. Here's a summary of the steps:
1. Open the document.
2. On the right-hand side, you'll see an icon of a clock; click on it.
3. This will extend a window with a history of your various versions. Simply click on one of the previous versions to restore it.
Please let me know if this does not answer your question. Thanks!