Are these types within the same project? If not, check the project references. They are referred through registry keys, which may have been changed outside the scope of the project.
Are you building a dll component or ActiveX exe (in contrast to a Forms exe)? If so, try unregistering the previous build.
If you have binary compatibility enabled in the project settings, VB6 will look for the previous build to resolve symbols. Maybe you removed the previous build, or it is somehow broken. Disable binary compatiblity and delete the previous build, then try to recompile. (Don't forget to unregister if it's a component, otherwise you'll leave your registry littered with broken class keys.)
[edit] Clarifications:
With checking the project references I mean the menu in the IDE. The list will say if it can't find a reference (if it was already marked as added) in the registry. Yes, those are the ones in the vbp, but with a text editor you don't really see if the GUID/version still exists. They might change if the referenced component is updated/patched.
With the registry becoming littered I mean that if you compile without project/binary compatibility then VB6 will generate new GUIDs for your classes. Public classes are noted in the registry with the path to the dll. Normaly VB6 unregisters a dll before overwriting it, but if you manually delete the previous build nobody is going to unregister it, so the class info stays in the registry, referencing a dll which no longer exists.
