Why has he shut me out?

Wow that's pretty cold. Try asking him but if he's not budging, it's best to try to move on (and you will in time). Sorry this happend to you. I don't think all guys are like that. There's no way you can generalize ALL half of the world's population down sadly to this or that, BUT you can distract yourself and then later on, the pain will lessen. You can do some self reflection and if you truly did not do anything, it will surely be his loss, not yours. Pick up your self worth/self esteem and march forward. It's the only way. Also, this is why we truly MUST know a person before really delving (especially giving your body and mind) and sharing it with them, in a relationship. Think about it this way, at least as someone told me, you are no longer wasting anymore of your precious time that you can use to do find someone better or do something that is more productive like take a class or do something fun that you've always wanted to do. Treat yourself. :-)

It's impossible for us to know.

Most likely, he met someone else.

But you need to step back a bit. You may THINK you'd feel better if he had given you a list of reasons why he no longer has those feelings for you, but it would actually be WORSE.

You need to accept this.

Have a few good rag sessions with your friends, then try to forget about him. Keep busy and distracted, until you are over him.

This is the kind of thing a guy will do who has something to hide. Were you to actually reach him, I promise you'd still not get a straight answer and would see things you rather not.

[Someone did that very thing to me last year - I found a side door and searched for him. He had a new girlfriend, pictures and statuses and messages on both fb walls confirmed my suspicions.]

Guys are crazy don't worry after couple days he will unblock you. Try calling him on the phone. Or use someone phone to talk to him