It happens to me a lot. For me it is completely situational and it is a stress or anxiety response. I call it situational mutism because that is an accurate description. It's not permanent. You don't have to worry about that. Sometimes it lasts a few minutes for me but I have had it last up to six months in one particular situation. When it happens to me, I don't worry about it. Because it is a response to stress and anxiety, worrying about it just adds more stress and anxiety. I just either write down what I want to say to others on a note pad or use ASL. Once I have relaxed enough and the anxiety and stress are reduced, my speech comes back on its own. I just expect everyone else around me to deal with it. I do not try to force myself to speak because that just causes more stress. I can also become mute when I am overwhelmed or overstimulated even if I am not anxious. It's the same response. Again, I expect everyone else to just deal with it and I just write or sign if I have to. But even then, I only write or sign if I absolutely have to. I try to just rest and not communicate at all if I can. Letting my brain rest helps me to recover quickly.
Situational or selective mutism, however you want to call it, is not something to be afraid of. I know it feels scary when you are not used to it because you feel like you have lost control and you never know how long it will last. But if you just know that it is a pretty normal thing for many Autistics, you can just try to relax and not worry and know that your speech will come back as soon as your brain resets it. Once you are no longer feeling the high anxiety or stress levels, your brain will reset your speech.
I also carry a little home made card in my wallet and I have in my Road ID alert bracelet profile and on my Autism phone apps that lets people know that I am sometimes mute because of my Autism. That way I can just show these things and people understand. I have never met anyone, except my mother, who has ever had a problem with this happening to me. My mother has her own issues but that's another story.
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"I'm bad and that's good. I'll never be good and that's not bad. There's no one I'd rather be than me."
Wreck It Ralph