What happened to my sister's Yahoo account?

My sister randomly got logged out, her browser wasn't updated, she didn't clear cache or data, or anything like that. She tried to log in, she knows exactly what her email is. But it said that they didn't recognize it. She swears she knows her email. So she tried to make a new account with the same username and it said it was taken. She tried again a few times, and it said the same thing. What the hell is going on? Why can't she f uckin log in? Also when she pressed "forgot username" it said oops bla bla bla. I doubt her account got suspended.