Cannot Tether iPhone Via USB

Hi

I have been tethering my iPhone via USB to my iMac without issue, it pops up in System Preferences under Network as 'iPhone USB'.

However, out of the blue, nothing to indicate what caused this, I connect my iPhone to tether, nothing. Simply won't tether over USB, my iPhone connect and shows up in iTunes, but nothing under my network preferences.

I ma using an iPhone 5 running iOS6 and Mac OS 10.8.2.

I can tether nay other way, but not wired! Any help please?

Thanks.