@delo,
There could be a million reasons why. I suggest you tell her how you feel. She could be stressed out, and it could be killing her mood. She may just literally not feel like it! Being a woman myself, I go through phases where I just don't feel like it for a while and sometimes put out only because I know it is unfair to my husband and I don't want the worry of him finding it elsewhere. Try helping take the weight off her shoulders. Cook her a nice meal, tell her how beautiful she is, offer to help with things you normally wouldn't. Just talk to her. Maybe you aren't catering to her sexual needs enough. Don't go straight into it, work her up and take your time teasing her and turning her on (without being too pushy) until she is dying for you to give it to her.