My fiance has not had period since may what could be wrong?

Many things: No periods can be due to stress, weight changes (too heavy or too thin), too much exercise, thyroid level being off, polycystic ovarian syndrome, or other hormonal imbalance (and pregnancy but that doesn't seem to be the problem here). She should see her doctor for further testing and a Progesterone challenge (pill to help her bring her period on).