Why won't my baby nurse?

Many possibilities. There are physical baby issues, mom physical issues, social issues, environmental issues - many possible reasons why babies might have difficulty nursing. See your pediatrician and/or breastfeeding consultant asap.
See specialist. Babies can have problems nursing for many reasons: difficulty latching on, inverted nipples, difficulty sucking eg cleft palate, etc. This cannot be answered without a good evaluation of both you and your baby. Good luck.