Do you know if she was on a birth control pill? Like maybe she upped her dose 3 years ago or something? My roommate lost her sex drive when she was on certain bc pills. Or, maybe she's depressed. DEpression makes people lose their libidos. Either way, I think that yelling at her about it will probably make her feel worse. Think of it like this: if you had an impotence problem, stemming from marital problems, and your wife guilted you for it, wouldnt that just make everything worse? wouldn't you feel like shit?
Sounds like she's probably got some deeper problems and this is an expression of them. I've never been married, but I can think of a few things that might be wrong.
1) She doesn't feel attractive to you. She may feel ugly or fat and be embarassed to be naked in front of you. Does she have self esteem issues? Did she gain weight during pregnancy that she never lost?
2) She resents you for some reason. Do you have a lot of foreplay? Do you *expect* sex from her no matter what? Maybe she feels used. Try kissing her or carressing her and just trying to make her come without trying to get your needs met. Spend time on getting her aroused. Maybe you can get her into it that way and once she's all worked up after a few times of this then maybe get back into the sex.
3) She isn't satisfied in bed. Ask her about her fantasies in bed at night. Get her mind flowing dirty...and then do whatever she says.
I would definitely see a marriage counselor ASAP. Work this out.