Step one, take a break from her. Go out, take a peaceful walk by yourself, and prepare for the storm.
Step two, get her out of the house. Distract her with something. Take her to a play, or a concert, or a chick flick, or just to fancy dinner. Don't be a cheap bastard, but I'm not telling you to spend more than $30 on her here, unless you /like/ going crazy with spending.
Do not shout at her. If you can manage it, listen to her feelings without speaking yourself. Don't try to solve women's problems; we hate that. Let her talk and she'll get it out of her system.
If you need motivation, imagine the sex you'll get in a few days when she's feeling happy and grateful to you.