compiler construction - Compiling .cc files?

guys i am having a weird problem it has been working fine but suddenly messed up
Here is the problem. I have test.h and test.cc and mainP.cc.

In mainP.cc:

#ifndef test_H

#include "test.h"
#endif

in test.h:

#ifndef test_H
#define test_H

in test.cc:

#ifndef test_H
#include "test.h"
#endif

now when I say test tInstance; it gives me undefined reference to test::test();
I checked my def. constr. is public . everything looks ok to me. I am not sure what i wrong? i use g++ to compile it.