Why is my friend ignoring me? How can I find out what I did to him?

okay….So basically I’ve known one of my online friends for about a year now….He’s really nice and stuff like that, but lately I’ve been trying to get a hold of him over the IM on MSN but lately he’s been saying “hey” and stuff but then I ask him something and he pretty much ignores it….Then today I was trying to get him in a specific mood, but he just said he can’t talk….I’m getting a vibe that he’s mad at me like I did something…But I don’t even know…What should I do? How do I find out what he’s mad about…?