Me and my friend have been the best of friends for years. We hardly ever fought. She is super sweet and hates if anyone gets hurt and stands up for anyone even though she is really shy. But recently she changed and she was super mean towards me and cruel. She says she wishes I died that she never asked to be my friend but I ignored it and treated her like nothing had changed. But yesterday she took my stuff and held it away from me refusing to give it back and I snapped. I said things that I never meant like that I regret ever being her friend and then we started arguing. And after it we stopped speaking to each other. Today I spent the day with my other friends but she was mostly alone. She doesn't have that many friends apart from me because she's so shy.
I regretted hurting her feelings. I'm really worried about her and I want to understand her better and understand why she's different and befriend that side of her.
But I was to afraid to make up. We never fought before and I don't know what to do. I'm terrible at understanding emotions. I just want things to go back to how it was. What advice do you guys have.

Communication.

The most impartant thing right now is to get involve you both in a good conversation. Ask her to go out one day and talk about all kind of superfluities. Then the moment is just going to improve to ask her why her behave is so different from the past.
But remember to just hear what she has to say, don't argue with her about the thoughts that comes out of her mind.
Just remember you have to wait a couple of days before you speak with her again, things have to calms down.
Good luck, and keep trying your best ;)