If your input is like "1/left", the track interleave is set to "mono" and what you get recorded is one mono track (not a "two lane" stereo track with signal on only one side) that you hear in the center (when pan is centered) you are recording mono correctly.
In that case the reason you're not being able to pan is something a little special.
The fact that you hear
both loudspeakers go silent when panning means you are
not recording mono on a stereo track (as suggested by some), because in such a case you would only hear sound on one side. But it means both your speakers/headphone cans are receiving the same signal (=mono). With my brains it means your signal gets turned into mono after it leaves SONAR (??).
My best bet is there's something strange in your audio interface routing or monitor cabling. So, to me this sounds like this far you have had all your projects in mono only, right?
Are all your tracks going through Master Bus in SONAR? Is there an Interleave-switch in the Master Bus (I'm not at my DAW now)?