To follow up on Graham's post...
The usual entries in the H/F toolbar's AutoText list are in header or footer
style. If you pasted text into your header or footer, it is possible you
imported a different paragraph style into it. Change the style to header or
footer and see if you have AT entries available.
If you do not, chances are good that you have overwritten your normal.dot
with something that does not contain the default AutoText entries. With Word
closed, try finding and renaming normal.dot to normalold.dot. Then start
Word and see if your problem is fixed. If it is, use the Organizer to move
any customizations from your normalold.dot file to the new normal.dot.
See
http://addbalance.com/word/movetotemplate.htm for step-by-step
instructions on moving / sharing / copying customizations including
AutoText, AutoCorrect, keyboard assignments, macros, etc.