Your first sentence implies that you have already opened Word or Excel and are choosing File Open to select the file you wish to open. Your next two sentences imply you are looking at the file in Windows Explorer and probably double-clicking that file. Which is it?
If it is the second, it could be either that the file extension (suffix) is incorrect or that the file extensions are not associated with the correct applications. If that is the case, see what you can find here
Search for 'reset file extensions windows 10'