I had this issue after plugging in an external speaker to the headphone jack. The speaker worked great but when I unplugged it my volume bar disappeared. When I plugged the speaker back in, it came back. I was going to clean off the dock but decided to just cycle the the phone off and then back on first and that did the trick. I wonder if cleaning the dock is the equivalent of blowing on the Nintendo cartridges, something to do while you accidently fix the problem.