in general a guy that likes you and is staring to often and continuiously may make me uncomfotable or just a guy that can't make up his mind about what weither he likes you or not or what he's going to do with you, or just someone who acts like a physcho stalker, though I kinda like don't mind that bit. :)
maybe she just found she likes you and it uncomfortable. or your an idiot douch, yes, both, and annoy that hell out of her, or are just a boredom king, who leaves her with a bad taste in her mouth.
Usually when I start to ignore a guy, its because I'm extremely mad at said guy. He usually did something that is completely against my morals or out of his character. If this is the case with you, you just need to be the bigger person and apologize but you can't just go up to this girl and say "sorry" and that's it. You need to tell her why you are sorry and that could be tricky because if you really can't figure it out your apology is just going to seem like a one word fix all kind of thing which does not meet most girls standards.
because she isn't interested anymore. I mean she probably tried to start something and had thoughts about what she likes about you and "what if" thoughts about if you two were ever to date. but she really did try and now she doesn't want to hurt your feelings because she know you like her.
or she could just be a complete bitch and stopped talking to you because she wants a new guy and isn't going to tell you. so you can figure it you when you see her with a guy you know.
to handle it, you just gotta forget about it for now. try someone new and if you have to even delete her from you cell [that is if you can still get her number from someone or somewhere]
But after like a month without talking just randomly text her some night and see how she has been and if she shows that she actually is done ignoring you then try again.