Your references are probably messed up.

This can be caused by differences in either the location or file version of
certain files between the machine where the application was developed, and
where it's being run (or the file missing completely from the target
machine). Such differences are common when new software is installed.

On the machine(s) where it's not working, open any code module (or open the
Debug Window, using Ctrl-G, provided you haven't selected the "keep debug
window on top" option). Select Tools | References from the menu bar. Examine
all of the selected references.

If any of the selected references have "MISSING:" in front of them, unselect
them, and back out of the dialog. If you really need the reference(s) you
just unselected (you can tell by doing a Compile All Modules), go back in
and reselect them.

If none have "MISSING:", select an additional reference at random, back out
of the dialog, then go back in and unselect the reference you just added. If
that doesn't solve the problem, try to unselect as many of the selected
references as you can (Access may not let you unselect them all), back out
of the dialog, then go back in and reselect the references you just
unselected. (NOTE: write down what the references are before you delete
them, because they'll be in a different order when you go back in)

For far more than you could ever want to know about this problem, check out
http://members.rogers.com/douglas.j.steele/AccessReferenceErrors.html