Error: missing an assembly reference... Need Help.

[RSB]

Hi every one,
all of a sudden my project has started giving me the errors about the
Reference. here is one example.

C:\Projects\myMainFile.ascx.cs(3): A using namespace directive can only be
applied to
namespaces; 'System' is a class not a namespace

or this Message..

C:\Projects\myMainFile.ascx.cs(4): The type or namespace name 'Data' does
not exist in the class or namespace 'MyNameSpace.ClassName.System' (are you
missing an assembly reference?)

Pleas help.

I have this Project in Source Safe...

Thanks
RSB