How can I deal with a friend like this...??

My friend is that type of person that when she gets mad or is just not in a good mood, she ignores everyone. She doesn’t speak and always has her head down.

I’m not sure whether to speak to her or just don’t talk to her at all until she is okay. But since we have class together, I don’t know whether to just leave after class without saying anything to her or should I at least say bye. Because I ALWAYS wait for her after class. I don’t want to feel uncomfortable because she is ignoring me.