Lost my bestfriend? please help.?

Yesterday, me and my best friend had a argument, because i went out with someone else instead of her, it was one day, and she didn't ask me to come out anyway. Now she is acting stupid and not even talking to me. We went back to school today and I tried smiling but she just pulled a straight face and stared at me, i tried to form a convosation when we were getting changed in p.e and when she put the wrong date in her planner, but it didn't work. But i did think i got a bit of a smile when i gave her the date for her planner, but she probably just glanced at me while she was laughing at something else. I really want her back as she is the best thing i've ever had.