She may have problems with her antidepressants  or birth control pills (which decreases sex drive) or she may be experiencing side effects from a pill she is taking. Depression is the main cause of not wanting to have sex. She may also have mood swings.

She may also find that sex is not very arousing or is very bored with it. I would suggest that you try something new on her. (new positions, sex toys, etc.). She might think that sex is nasty and is somewhat old-fashioned. (a "prude") and may not be used to sex. You should explore new options with her.