I think your wife should have some blood work done to see if her hormones are at the right level. Have you nicely told her you would like to be together more? As Shangrala said - she might not be feeling good about her weight gain or she might be mildly depressed.

After ruling out physical reasons, I would try counseling. Explain to her - that you need to feel she desires you, just as you desire her. Of course you never want to argue or let resentment creep into the picture, since this is a sure turn off for both of you. Best of luck.