I met the girl 2 weeks ago during work. We worked together, made fun of each other. I think we both enjoy it very much. During that time, when I talked with her , she would smile or giggle for no reason.

I asked her to hang out with me (for dinner) after the two weeks. She refused (she said:"I don't have time right now. I will let you know when I can."). And, now when I talk with her, she seems to be puzzled, and no longer smiling.

I wonder what the situation is, and what I shall do since I still meet her everyday during work.

girls can smell desperation so don't let out a whiff of it.
It sounds like you guys had a pleasant exchange a while back and either your taking her friendly attitude as a false IOI or she actually is open to the idea of you.

by the sound of that latest exchange however it sounds like you may have overwhelmed her and she isnt trying to be with you in that context.

i think the best thing you can do is continue to be nice and friendly and outgoing around her but act as if the denied dinner date never happened- you have to act like you are in a suit of armor and that refusal is just a pellet skipping off your armor. it will be attractive for her to see that your confidence is unscathed and you havent skipped a beat.

you cant just "hang out" with a girl by taking her to dinner- thats a formal date thing to do.

you should hang out with her in a friendly no pressure context first- tell her you were gonna go to the mall/downtown to stroll around and do some windowshopping but your buddy flaked out on you, then ask if she wants to take his place.

with girls things work better if they are under the impression that things "just happen" so keep hang out dates non formal and very spur of the moment- as if you really were just asking your buddy to go with. while your out with her just hanging out you can stop for coffee to plug a real date scenario into a nonthreatening hang out setting.

dont think of girls as things you have to treat *this* way to get *this* response- outside of how they pee and what they wear when they wanna go out and get screwed girls and guys arent that different