Compatibility mode is "automatically" activated when you open a file that is
in something other than a .docx format. If you can't open the file in the
first place it suggests that the file is either *not* a readable file type
(something other than a text-based document) or may be corrupted.
If Word won't open the file normally, as a last resort you can try using the
Recover Text from Any File option in the Files of Type: list at the bottom
of the Open dialog.