Audio Systems: Sony Receiver STR-K660P, channel analog, sony receiver

Question
About 3 years ago, my wife and I bought this stereo/surround sound system. It's been working great all these years. 3 years later, we can play the CD player through the system, but when I switch from "Video" where the CD player is connected, to "DVD" where the DVD player is connected, the display reads "DVD" then immediately flashes "UNLOCK". I have no idea what this is or how it got there. Can you help me remove this so that I can start listening to the DVD surround sound again? Thank you.

Answer
try unpluging the power cord from the sony reciever for 24 hours to see if that resets the unit.
Let me know if that helps.
Also, how is the dvd connected to the sony?  Is it just the left and right audio, the 5 channel analog, or optical or coax digital?