It's my friend at school, we both used to hang out together, talk, and text, a lot and we were pretty close, we even held hands, it was like she was my girlfriend but it wasn't official.. We both knew that we liked each other, but 2 weeks ago she stopped talking to me because she said something was going on.. so I wait and during this time I'm really depressed and sad. Recently, she put a note in my locker saying that she's in a relationship now.. now I'm ******* sad but I can't let her know that I'm sad because I don't want her to worry.. She doesn't know that I still like her.. but I do..

We still talk and text a little, but we don't hang out together anymore and its like a ******* wall is blocking everything, like I can't tell her anything now because she's in a relationship. Even though her boyfriend is like half way across the country in another state, it still feels like I can't tell her anything.. Apparently the person she dating dated before, but he cheated on her.. and she still goes back to him..

I still like her but I can't tell anyone because I don't want to break up a relationship she's in... I can't tell anyone, but if I could, she's the only one I would tell, and I would tell her how I feel about her...

I just don't know if this is the right thing to do.. I'm willing to take the risk of telling her, but what if I get rejected? We won't be friends anymore right after that? I don't know anymore.. please help..

No, She has made it clear that she is in another relationship so your relationship has now changed. If you tell her how you feel (I suspect she already knows), you may damage whatever friendship you have left.

So your best bet is to continue simply as friends. You need to move on to your own relationships.

As a side note, I suspect the *** in your post was the masking of profanity that you used. This site does not allow such language. And, In my opinion, such language has become way too common.

Well, go and tell her how you feel, you never know, if her new boyfriend has been dishonest, maybe he's made her be his girlfriend, you know, he blackmailed her. Anyway, she'll probably tell you what's going on, you two will still be friends, she'll just know how you feel about her, and you know what? She'll be happy that you were so honest with her. PokeXpert

She has a boy friend, there is no reason nor do you have the right to infringe on that relationship.
If she is your friend you don't do it because friends don't do that to each other.
Time to move on.