Hello, I'm having a similar issue to what another member had a few years ago.
My TV is an RCA 20F420T CRT, manufactured in August of 2006. I entered the Service Menu to fix some issues I had with the picture, and now I have no sound coming from the unit and the service menu refuses to go away. I've tried powering off the unit, power cycling the unit, and even unplugging the unit while it was on, all to no avail.