This particular AutoText entry seems to have a tendency to go walkabout.
Note that it is formatted with the Header and Footer style, so make sure you
are looking for it there. If it is really gone, it is easy enough to
recreate.

1. View | Header and Footer.

2. In the header pane, type "Filename \p" (without the quotation marks).

3. Select what you have just typed and press Ctrl+F9 to turn it into a
field.

4. Press F9 to update the field.

5. Select the field (being careful not to include the paragraph mark) and
press Alt+F3 to open the Create AutoText dialog.

6. In the dialog, type "Filename and path" (without the quotation marks) and
click OK.