You see a warning message similar to: Cannot open the file “<your file name>”. Adobe Indesign may not support the file format, a plug-in that supports the file format may be missing, or the file may be open in another application.
Cannot open the document “< path to your file >”. You may not have permission or the file may be open already.
For the most part, these two messages are telling the truth. Are you trying to open an InDesign file of some sort? ID does not “Open” images, word processor files, spreadsheets, PDFs (without a plugin), etc. These formats must be imported using File > Place… In addition, if the format is one that WILL open, but file is in use, there will be a lock file (.idlk for .indd files) in the same folder. If InDesign happened to crash or was force quit with the file open, that lock file probably was not deleted. Make sure you really do have permission, that there is no lock file, and that you are trying to open something ID is really able to open. If you get this when you try to open by double-clicking a filename, check to be sure the file shows up in the list when you try File > Open… If not, that ‘s a clue it’s the wrong type.
The other possibility is the file is corrupt. This can happen when files are transferred via email without being compressed for protection, or, occasionally, when they are stored on a network or removable drive and there was an error during a write operation. If you suspect file corruption, please start a thread in the main forum and someone will try to help you. Provide as much detail as you can about your version of InDesign, the OS, and the file history, as well as what happens when you try to open it, and be prepared to share the file for analysis and possible rescue.
Strictly speaking, this message is also accurate, but in a very obtuse way. The plugins need to be updated because the ones used when the file was saved are from a later version than the one being used to open the file. You cannot update the ones you have because there are no updates to be had (except by installing the newer version). You need to open the file in the correct version or later. Or if the file was provided by someone else, they need to open it in the correct version and export an interchange file for you. (See: http://forums.adobe.com/message/3838856#3838856)
But, you say, my computer says the file is the version I have! That information is passed along by the Operating System. Your OS is dumb; it has no idea about what other versions of an application may be out in the world, only the ones on your computer, so it tells you it recognizes the file and that it must be of the same version that you have installed. This is not Adobe’s fault and they can’t do anything about it.