Why does she leave me on seen ?

I used to talk to this girl. We had amazing moments together and we got along very well. She was the peffect girl for me. We had so much in common and everything! Thing is we slowly stopped talking for idk what reason. We would talk every now and then but not as much and the way we used to before. I miss those moments. I messaged her today "hey! (:" she replies "hey" and I ask " how ve you been ? (:" and she left me on read. I m so heartbroken by the fact she did this and I m confused. What did I do wrong ? Any ideas why she is acting this way ?