Call me old fashioned but communication? When a sex problem occurs both of you need to be able to have an honest and open conversation. It could be she's lost her desire to have sex with YOU e.g. the sex got repetitive for her, however it could be that she has lost her desire for sex in general. Perhaps her sex drive has been affected by some medication. Maybe she put on weight and isn't feeling as comfortable in her body. Maybe she is stressed at work. Maybe she has started a new physical activity. Maybe her diet has changed. Maybe something traumatic has happened in her life.
In the past my sex drive has been affected by external factors and I would hate for my lovers first reaction to be to go out to flirt with other men to show she had options. Come on guys, we are talking about a LTR here! The girl you are closest to. Take the PUA hat off for one friggin second.
How much non-sexual touch do you share? Baths / showers together, cuddling to watch a film, massages (not as foreplay for sex). Walking hand in hand. All of this releases oxytocin which keeps bonds between lovers strong. It's likely that if you feels close to you she will want sex.
The other consideration is that you have unfortunately reached the end of the neurochemically charged honeymoon period - see
http://www.reuniting.info/science/why_d ... ywed_study.
If sex has got repetitive, predictable and dull, you can add novelty (think sex toys, kink, a third lover) to spice it up. The problem with this though is that you become acclimatised to each new novelty and have to add more next time to get the same effect. Like a drug user chaser an ever greater high you eventually burn out.
An easy solution is to seek a novel partner, which instantly appears exciting - see the Coolidge effect -
http://en.wikipedia.org/wiki/Coolidge_effect . It's the reason so many people end up cheating or if they have some morals become serial monogamists. Another option is to explore other forms of non-orgasmic sex such as Tantra and Karezza (n.b requires an open mind) which trigger the bonding programme rather than the mating programme. Having sex without movement with your partner is like a meditation. You get something very different out of it and as such it can be used alongside regular orgasmic driven sex. It may be your partner is after this sense of connection and bonding that regular sex isn't giving her.