That girl suddenly stopped talking to me.

First, you should correct it. Next time reply to someone else's message, don't be busy. It should be said that we have something to do now, so we can talk about it later, or if you have something, don't reply to the text message yet, and then reply when you are busy. She should be angry with you because she ignored you. You shouldn't say anything to stimulate her. You should sincerely apologize. Now, I think you should send her a message. Don't be angry, and say you'll see her again in a few days. Wait a minute. You should still be angry now. Find her in two days. Good luck.

That's the feeling. It's hard for us boys to read girls' minds. I feel she is still angry, or she has told herself that she hasn't talked to you for a week or something.