What can cause a Variable Type to disappear

One of my workbooks won't allow me to use the VBE variable types (VBProject, VBComponent, etc.) They aren't on the popup list that appears when I type " as ". If I just type in "VBProject", I get a compile error "User-defined type not defined". I have to define the variable as object, and the macro still works. I do not have this problem in any other workbook.