I have the same problem every so often. Seems to stem from using the fast forward or back buttons on the steering wheel. It normally goes off again after a while, 5 mins to 1 day!..after pressing the same buttons again a few times!
Any "techies out there that can say why it happens?. talked to a radio repair person and looked at the manual, and it shouldnt even do "mute", unless it is a hangover from the mobile phone wire in?