What causes a young adult to suddenly stop speaking?

One thing you might want to look into is elective mutism (edit: apparently it's also called selective mutism). It doesn't even need a trauma. Or possibly a social phobia of some kind - fear of strangers makes her not want to speak to them. (Which also doesn't need a trauma; I was like this in high school. I couldn't even order my own food at McDonalds because my voice would just close off even if I got up the courage to try)

Dare Points: 11
Doctor Who references: 3
Mad pixie giggle fits: 3
Times Jack has died: 0 (still! This must change soon)

She could see someone die or be mentally/verbally/physically abused. She could even be threatened by someone or something. I don't know how far out you want to go but maybe a hallucination of something tramatic could do that to her.

Hope this helps!
Good luck!

She could choose to do so as a vow, like the brother out of Little Miss Sunshine. He swears not to speak until he gets into the Air Force.

I think it's a lot more plausible for a young adult to decide to stop speaking, as the above poster suggested, than for them to be traumatized into it. Muteness as a result of trauma is incredibly rare, and a really overused trope in fiction. What if she gets angry at her family, or some particular person, and vows not to speak ever again? A silly, petulant thing to do, but what are young adults if not silly and petulant?

I went through something like that once. Not for very long, I don't even think it lasted a full day. My mouth just felt gummed up and thick, and I was inexplicably angry at everything, but very hazy and listless. For the majority of the day I was genuinely afraid of what might come out of my mouth if I made an effort to speak, so I just didn't. I remember my friends were very confused and hurt because they thought they had done something. I think someone finally caught me off-guard and I answered some kind of very basic, everyday question automatically, and then I was back and it was fine.

I guess what I'm saying is that it doesn't HAVE to be anything particularly epic, like rape or a death in the family. Many people feel traumatized or overwhelmed just reading the newspaper. Sometimes people just suddenly don't feel the effort is worth it. Sometimes they genuinely won't know why they feel that way, and it's so confusing that it makes them even more afraid. I think you should keep this in mind as you explore the possibilities.

Do you want it long term?
If so, it's usually because of a trauma or a mental disorder.
As a child, I almost never spoke until second grade or so.
I did speak occasionally, but it wasn't very often, that I can remember.
I have generalized anxiety disorder, social phobia, and agoraphobia.

Maybe it could be something like that?
Mental disorders can develop and worsen over time, and they usually don't strike until teenage years.