sounds like a faulty usb port, seen this a couple of times, its like it just locks into usb mode, and when theres no usb in it just goes to standy mode.
I presume when u power up/down you can hear the stereo booting up?, also do you still get 3 beeps when key out?
...if so I would presume that there has been some debris in the usb port which has jammed it when cable was plugged in, if so easiest thing is to just try plugging in new facia panel and see if this does the trick. Then you know if thats whats at fault, if halfords stock the unit id give them a bash. if you find out the model no i can tell u straight away