If you want her back, get her jealous. Tell her you are moving on and going on a date, and do it. Don't call and see what she does. Always be happy when you talk to her and never let her see you down.
If this doesn't work, nothing will, and there's a good chance she has someone else. This is usually the case anyway, she may just be acting depressed to try and save your feelings by not telling you the real truth.