I wasn't thinking clearly in my first response. I was envisioning you removed the faceplate and button assembly since those are the ones that commonly get rubbed off, but you can't buy those through the dealership. I can't even get them anymore since GM stopped allowing us to buy radio service manuals and parts after the '94 model year. They locked up all the repair business for their two grossly-over-priced repair centers. Ford pulled that selfishness after '97.
If you simply pulled off a round turn-type knob and pushed a new one on, that should not have affected the radio. The engineers built the Body Computer into the radio starting with some 2002 models. If your chime still works, the radio is most likely okay, but we have to figure out why it won't turn on. If you have to push that knob to turn on the radio power, it is possible that control is mounted to a circuit board that got cracked. I've worked on a bunch of radios that were dropped by the UPS basketball team, that had that design. Any hard impact to the power / volume knob pushes it in too far and it hits that circuit board and cracks it. That can result in no change in volume and / or you can't switch between "off" and "on", or you can't switch between station display and time. I would be real surprised if you were able to damage the circuit board by just changing the knob, because if it was built like that, they'd have problems right on the assembly line.
I'm afraid I don't have a better answer. I haven't worked on any GM radios that new so I don't know how they're built or what might have happened. Just to not overlook anything, you might want to check the fuses inside and under the hood, in case pushing the knob on pushed the circuit board far enough for a point to short to ground momentarily. There are likely to be three or four fuses all affecting the radio / Body Computer, and some or all of them will not be labeled "Radio". Beyond that, you'll have to consult the dealership.
Thursday, February 12th, 2015 AT 2:03 PM