Nope, she's just passed the stage of "Hey, he's caught my eye, he's cute..." to FULL BLOWN CRUSH and she doesn't know what to do around you so she's freezing up. Strange as it seems, it's natural and human nature. She doesn't want you to see through her but she still wants you to pay attention to her!
That is, if everything was going okay, she was single and didn't say anything rude to you before she started ignoring you!
DO NOT TEASE HER! It will clam her up more and make her afraid of you. Start out with a simple question. "Hey did you see that crazy wreck on the way in to work/class?" or "So and so seems really angry today. Watch out for them. Do you know what might have happened?" Give her something easy to answer and ask you simple questions about back. (Even the most frightened girl can give a one word answer and at least look expectant, hoping you'll respond!) Try not to ask her too many direct questions about herself. She's feeling too self-conscious at the moment and even a question about where she got her new purse will have her staring at you like a deer in the headlights as if she did something wrong and then she'll mentally disect the question & her answer for the next couple days until she's SURE you don't like her and she'll vow to not embarrass herself and will ignore you even more.
As hard as it is, try not to let her shyness-driven ignoring bother you until she makes it CLEAR she wants you to leave her alone. This is where the majority of confusion comes from I think. You guys don't realize when we get to the level of developing a crush on you we suddenly tense up and go quiet when you're near. You'll know this is true if she seems to be fine with her friends, you hear of her saying hilarious things & joking around but she's a completely different person as soon as you appear. This is a GOOD SIGN! It also means she's shy deep down so be extra kind towards her. A gentle tone and kind expression that tells her you won't think anything she says is stupid will go a long way. :)