It doesn't happen very often but when it does, it's triggered by something stressful.
My brain seems to go blank and I can't find the words I need. I start slurring the words that I can find and then eventually am not able to talk at all.
When in this state I can still communicate by text or written, it's just verbally that I can't communicate.
If I sleep, I'm verbal again when I wake up, otherwise it can last anything from 30 minutes to a few hours.