I would suppose that you saved something else with the extensian .INDD than an InDesign file. Either another file type or a corrupted INDD file.
Look into the file with a text editor to see if this is really an INDD file, compare the size of the file it it is around the same size as others with similar content.
Make a copy of that file, exchange the extension with other possible extensions like IDML, PDF, EPUB, and others.
Try my script in http://forums.adobe.com/thread/934429 -- it will tell you if it's an InDesign file or not.
If it tells you it is, the file must be internally damaged beyond repair. (And then you've lost it. Yes, it happens.)
If it tells you it is not, then ... well, then it is not a native InDesign file. At that point you need to retrace your steps: what did you do when you thought you were "saving" it? (As it is, in fact, impossible to use "save" in InDesign and not save 'as' an InDesign file.) What other file type could you have used? Do you ever manually or automatically rename files?
Needless to say, if it is no longer an InDesign file (never mind what the file extension says), then you cannot open it in InDesign anyway. But if you know what type of file it is, you can open it with the appropriate software (e.g. a JPEG viewer or an RTF reader) and see what's in it.