Nipple inversions have many causes - and some have no apparent cause.
Most nipples can be protracted through brest feeding - a baby latching on will draw out the nipple. Ofcourse, in some women the nipples are permanently inverted and irreversible (unless ofcourse, cosmetic surgery is performed). Even in this case howver, breastfeeding is possible since the infact sucks on the areola.
Breast pumps and other suction devices may also help to draw out inverted nipples.