Many possibilities, but of few of the most common are:
1- You have a SET PATH statement in use, you are trying to open a file which is not locateable within that path.
2- You are trying to open a file that does not exist at all
3- The directory has insufficient rights.
4- The attribute has been changed by another app to HIDDEN.
5- You have spaces in the path/file name and haven't surrounded them with quotes: USE "C:\Program Files\MyApp\Data\Mytable"