-ちゃった is a contraction of -て しまった. So it is やって しまった which means "I went ahead and did it" or "I ended up doing it". (やる means "do", and しまう to finish, or do something once and for all.)

Tony-san has already given you a great answer. I just want to add a little. Sometimes, やっちゃった is used when you failed or made a mistake. In this case,　やっちゃった　mean 失敗しちゃった. I think this is a slangy expression. X:やっちゃった！ Y:どうしたの？ X:テレビに夢中(むちゅう）になって、フライパンを焦（こ）がしちゃった。 （As I was absorbed in TV, I have burnt a frying pan.)