Before even doing what Whopper suggested, check the connections and ground at the starter and the battery. If a connection between the battery and starter is bad, the solenoid may not be getting enough power to fully engage the starter so it just free wheels instead of cranking the motor.

Another quick and easy thing to try is to disconnect your battery for a minute or so to reset the computer. Check to see if you have any codes and record them first before doing that. I had an issue recently with my 2008 SS where all of a sudden turning the key would not crank the motor or even engage the starter at all. It was like I didn't have the clutch pedal in and the computer wouldn't let the engine crank. It happened right out of the blue. I pulled up to a place of business, shut the car off and went in for a short time. When I came out, the motor wouldn't crank. In the end, simply disconnecting the battery and resetting the computer fixed the problem. It wouldn't hurt you to try doing the same thing and it might just help.