How come the little hummingbird abandoned her nest?

I have a little hummingbird nest and she laid two eggs in it. One of them hatched about a week ago and the other didn't. She has been feeding it regularly and is always at the nest. Today, she is nowhere to be found. The last time I saw her was yesterday and she was feeding her baby. What should I do?
Update: Also, she's right outside the window of my office so I always see her (I can't miss her because I work at home and am always at my computer) but she hasn't been there all day which is VERY rare for her.
Update 2: I just checked the nest since she hasn't been there all day and the little baby is dead. =0( Thanks for all your answers.
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