Women's sex hormones are very susceptible to stress. She's not necessarily depressed, and the relationship isn't necessarily over...

Think about it like this:

From a biological perspective, sex is for procreation. Women's 'investment risk' in sex is much higher than men's because a baby means 9 months of discomfort, possible death in childbirth, and far more caregiver responsibilities (traditionally). When the environment is not right for a woman to have a baby, we have certain physiological processes that reduce our want to have one (read: sex drive). Some women are more influenced by this than others.

Cortisol is the stress hormone and too much of it inhibits the production of oestrogen and testosterone in women. This causes a reduction in sex drive, among other things.

As a woman, if I'm chronically stressed, the last thing on my mind is sex