One other possibility that occurs to me is that you were saving on a flash drive. (Not a good idea.)

Only Word can change a document to a template using the SaveAs command. (It might be possible for a very skilled programmer to do it, but as Paul noted, simply changing the extension in Windows does not do it.)

Also, should not be "opening" the template but rather double-clicking on it or using it through File>New to create a new document.