This topic can be tackled from different angles. As you know, for married couples, intimacy and sex is a build up of emotional and affectionate interaction between the husband and the wife. It is not just about relieving oneself sexually. Are you tending to her emotions, and making her feel valued, and giving her time, and being at home.
Are you neglecting her, or treating her in a demeaning way, or abusing her, or are you promiscuous and getting involved with other women?
How is your joint sleeping pattern. Do each of you sleep in different times, or separate from each other? Have you abandoned your bed, and do so for long periods of time?
These are all scenarios that could lead to this underlying problem of lack of intimacy. Of course, it is also a build up as well, and will not just happen suddenly.
If the wife is not interested in sex, it could be because of a variety of reasons, either stemming from personal issues, or underlying marital problems that are not being resolved. It could be hormonal imbalance, or mood. etc.
Have you and her had a serious discussion about this. It would be advisable to see a specialist or a psychologist, or a female marriage counselling.
In any case, if you have reached the stage of frustration, and did not wisely deal with the situation, and avoided solving the problems, then you have few options.
1. It is wajib for wife to do tamkeen, which is for her to comply with husband, in a healthy and positive way, as long as there are no other legitimate reasons to reject.
2. If none of the above issues are applicable, and she does not want to solve things, then you must either get intervention from her family, be patient, or as a last resort, get divorced.
3. I would recommend you find other means to "relieve your frustrations", because that would make your marriage and your relationship with your wife even more complex.
Hopefully this quick answer will shed some light on how to look and deal with the situation you are in.
With prayers for your success.