Though I never did follow Dilbert creator Scott Adam's blog regularly, I recently discovered that he suffers from an exotic ailment which effectively causes one's brain to forget how to talk -- Spasmodic Dysphonia. It's not like the vocal cords just stop working, or even that the brain loses all contact with the mechanism of speech. Many sufferers of Spasmodic Dysphonia can frequently sing or get their body to produce voice in other ways. The brain appears to go about the process of singing or oratory speaking differently than everyday speaking. So the sufferer can be unable to speak normally in everyday situations, but be able to give presentations while using stage voice.
The interesting thing is using just this knowledge, Scott Adams tried -- through repeated trial and error -- to figure out a way to reteach his brain how to speak, and has found success.