I found an answer here: http://forums.adobe.com/message/2604646
- I opened Windows Explorer and went to Tools > Folder Options... > File Types.
- I selected INDD and clicked Advanced.
- I selected the Open action and hit edit.
- I browsed for Adobe InDesign (for some reason Adobe doesn't appear in Program Files by default but if I navigate to Program Files and then type Adobe in the file name box and hit open then I can browse the Adobe programs).
- I selected InDesign and hit Open.
- Then back in the editing action window I added "%1" to the application used to perform action text box, so that the whole thing read: "C:\Program Files\Adobe\Adobe InDesign CS6\InDesign.exe" "%1"
- I entered indesign in the Application text box and then clicked OK.
- InDesign CS6 will now open INDD files. At last!
^Right. And for clarity, it bears mentioning that Helen's problem, apparently, wasn't that .indd files couldn't be opened InDesign CS6.
In fact, the only problem was the file extension association was lost at the OS level, so .indd files couldn't be opened in InDesign CS6 by double clicking on the file.
Helen's steps 1 through 8 outline the process of restoring the file extension association in Windows. Considering that it worked, it stands to reason that all along, the .indd files would have opened in CS6 by launching the application and opening the files via File > Open...
It probably bears further mentioning that on Windows if the file association is lost it means the icon handler .dll file is damaged. While Helen's solution will restore the file association, it will NOT fix the icon handler, and all .indd files, regardless of the version they were saved in, will appear withthe CS6 icon. To do a proper repair you should reinstall.