As someone with autism/speech delay I have to say that while I'm aware there is a whole spectrum of different severity and variation, I've never heard of autism itself being the cause someone to go from speaking well to going completely mute.
While I'm no doctor, are you sure it's not perhaps an overlapping cause like selective mutism for example, a severe anxiety disorder? Autism is a lifelong thing that effects their communication skills, perception of the world, and how they relate to others, whereas anxiety disorders such as selective mutism are caused by severe anxiety and may not be lifelong/is treatable. It could be overlapping in that I know from experience autism comes with it's share of extra social anxiety, and if that it were severe enough it could lead to anxiety disorders.
But again, I'm not a doctor.
