Just a stab in the dark, but did you rename any of your tables? I've have bound combo boxes then renamed the source table and guess what.... they didn't work. I think the error you site is the one I got.
To fix (and sorry if I get some of the terms wrong, I'm still pretty new to Access), go into the combo box properties and change the row source, I think it is.
That's my "shot in the dark"... Otherwise, Jack is right, more detail please.