I'm not trying to be mean, I promise. But this is so highly unusual that I am curious as to how this could happen. The baby drank off mom for 3 weeks then nothing, all of a sudden? Could you please get a picture of the mom's udder for us? Have you tried holding or towing the mom up for the baby to drink? Have you attempted to milk the mom at all? Maybe mastitis, that would make the mom uncomfortable enough to not let the baby nurse, but there is still milk in the mother's udder... and the mastitis itself would need to be addressed if this were the case.
Pretty please on the picture? I run dairy goats of my own, and am an Animal Science student, hoping I can help you out a little bit.