Imo, to address the issue of Iambren's post, the manner of communication matters. Imo, his post sounds harsh to women, and it is not all that accurate when it comes to relationship. Yes, men and women have an obligation to each other, but that does not mean it's up to HER to find a resolution to HER problem. In all honesty, SHE doesn't have a problem with low libido. Rather HE (the OP) has a problem with his WIFE's low libido; therefore, it becomes a MARITAL problem for BOTH to resolve.
As has been mentioned, low libido has a whole lot of reasons including hormone issues (time of month, time of life, etc), medical (depression or side effect from a medicine, etc), genetic (just the way she is), situational (every marriage goes through their down-times), relational (maybe she needs more connection with you on a non-sexual level, more touching, more sharing, more openness, etc). Imo, the best thing you can do is make time to talk about each of the possible causes, and once the cause(s) have been identified, find out how to best address them.
A very important thing to remember in this though, is that if she feels for a second that the marriage is threatened because of this and it's up to HER to fix it, she will then likely feel resentful, making her even more turned off than she already is. In all of this, it is up to both of you to communicate respectfully and deal with this together in love for one another.