What I do to not get in this situation is I make the relationship very sexual, but in a smart way. You don't get anywhere by saying "I'm horney", your wording should be "You make me so horney!" or "When you walk around in that outfit, you just makes me so hard.." Implement this with a strong leadership and it should help. So if the problem is that she doesn't feel sexy anymore, this can help. Also, if you're bad in bed (by making it all about you), then she doesn't feel like having sex because it feels like a chore. Try asking if you can go down on her, because you like to watch her have an orgasm. (Don't have to be true, but it could help. Personally I love watching women scream out of their lungs because of me.) You didn't give that much information, so it's hard to say what the reason is. But most times it's because the girl feel neglected in the bedroom, you don't lead well or she has a problem with her self image. Telling her she's pretty or beautiful won't help, you have to make it sexual. The drawback of this is that she can get the feeling that you only think about sex, if you take it too far. There's a lot of articles on leading and how to be good in bed on this site, so you won't have a problem finding information on that.
I would at least give this a try before dumping her if you really like this girl. If all fail, leave her. There's enough guys not having sex in relationship, don't be one of them.
-TC