I dunno... it's hard to say. did you guys get in a big fight or something? if so, maybe write her an apology letter.
I think if you really want to write to her, you should send her one big email with all of your feelings in it instead of a ton of little messages. that will show her that you genuinely care about what is happening and how much you want her back.
it all depends on what went on between the two of you, so I can't really be much of a help, sorry
if you still have a hope that she will like you again, write her this email. even if you dont have hope, you should still write it.. but make sure you try to be realistic in your mind about whether or not she will accept you again (im sorry if that sounds harsh.. i know it's really hard to do, but it's really painful when you try to believe that something will be good again and it never gets better)
I think that you should write this email, then give her maybe a week or two to respond, and then if she doesnt.. you have to try and get her off of your mind -- i know it's really hard... i've been there before.. when I thought a guy really loved me, but then he ditched me for whatever reason. i gave them so many chances, but they threw them back in my face. it's really, really hard to let go of someone who you love so much like that, and it takes a long time..
but i truly wish you the best of luck