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I called, but she didn't answer her phone. When I went down to the apartment to drop her roommate's keys off (I figured she'd want them back) she shut herself in her room and wouldn't come out.

Send her a letter or a mail to explain her what you are feeling and ask her why she does not want to talk to you.
If she does not answer, ask to her roommate or one of her best friend why she left you.
Maybe there is a misunderstanding but maybe also she has met someone else or she is a girl without a heart. (Yes it does exist !).

If there is nothing to do, i hope that you will forget very soon this girl and meet a new one.

Barta