For some reason, the nudge keys on the numeric keypad quit working, as has the S (split track) command. I checked, and they're still assigned/bound to the keys. Is there some master key I might have accidentally pressed (again)?
 
I can reassemble the tracks into a new project and I don't have the problem, so I'm assuming I pressed something on the keyboard to disable them.

Num Lock perhaps? Not sure about your 'S' though. 

Num Lock is on.
 
I closed down Sonar, shut off the interface and turned everything back on. Works normally. Go figure.

Check that the edit mode of the track is set to 'clip' in the drop-down menu. Certain other modes will stop the split function from working.
 
I notice you have an understanding wife in your equipment list. Just wondering if that particular feature works consistently? I too have one, but once in my studio seem to have a great ability to overuse the understanding feature until it no longer seems to function at all:) User error no doubt!

If you have built in keylogger protection enabled with your antivirus that might be an issie.

Hitting 0 on the numkey pad toggles "Enhanced Keyboard Editing" mode which converts nudge keys to navigation/editing keys. S was probably something else coincidental.