Very few people like my critiques, but I continue to give them anyway.
Low key images can be very dramatic and dynamic, but just like any other photographic technique it must be done properly. The first problem in your image is underexposure. Contrary to what most people think low key images are NOT underexposed. Any time a significant and/or important part of the image has an "R" reading in Info in Photoshop of less than 32 that area is underexposed. Most of the dark areas in the above image (her blouse, her hair and the right side of the guitar) are at 7. To give you an idea of what a 32 looks like, the small area below the fret board, in front of her right hand just behind the highlight on the edge of the guitar measures 32. It is still dark, but not so dark that there is no detail. Our eyes skip over areas that have no detail because there is nothing to see. Why look at blackness. Now if the dark areas have some detail in them our eyes will investigate those dark areas and any time an image causes us to stop and investigate it it is more interesting. The cure? A fill light above and behind the camera at about three to four stops under the main light should put you in the ball park exposure wise.
Composition. I'm not sure why there is an area of nothingness at camera left and the fret board and her left arm are lopped off on the right side of the image. Had the camera been moved slightly to the right and backed off slightly the composition would make more sense and the missing fret board and arm would have added strength to the composition.
Benji