Instead of ignoring her back, or passive aggressively shouting "Hello there!!!", acknowledge the fact that though you don't know why, you do know she doesn't want to speak to you and she doesn't want you to speak to her.
So just walk past her normally and go about your business. Don't avoid her, or seek her eye for the opportunity to say hello again. Ignoring is actively not acknowledging someone, stone faced, or as if she didn't exist. So don't do that. Just walk past respectfully with your mind on your own business, on your on day, to where you are headed, literally down the hall and figuratively through that day's routine.
You've asked her via text and she didn't reply. That is maddening!
You've done all you could do if you truly, truly have no clue why she is doing this.
The only justification she has for this is if you've done something very offensive and she's tried to talk to you about it and you refused to admit what you did.
Or if you offended her or hurt her somehow, and it's obvious you know exactly what you did but are pretending you didn't do anything wrong and are pretending you don't know why she's ignoring you, this is the point at which a person feels they have no choice but to ignore i.e. the person in the wrong has had the chance to take responsibility and is not, or worse is blaming the other person, or pretending nothing happened and are like "Why are you not speaking to me?" all innocent.
This is not what you're saying though. You say you have no idea. What was your last conversation about? You said she used to speak to you. Can you give us more information?
I've had this happen to me, sometimes I can figure out what it is, sometimes not.