If everything else worked apart from Sonar VST's it won't be a cabling problem, unless you were just using MIDI with Sonar and needed MIDI in/out through a cable. It will probably be back, anyway all the advice is in the thread if it should happen again... Cheers...

When you unplugged everything, did you unplug the USB cable to your interface as well? If so, that rebooted the interface. Sometimes a software reset isn't enough.
 
Regards,
Dan

Doktor Avalanche
If everything else worked apart from Sonar VST's it won't be a cabling problem, unless you were just using MIDI with Sonar and needed MIDI in/out through a cable.

I agree. By the time audio is being passed to the interface driver, VSTi output is already mixed into the stream or not. It doesn't make sense that a driver or hardware issue would affect only audio from VSTis unless the MIDI was coming in from somewhere else. I would wager it's more likely the associated reboot cleared some bad pointer or other corruption in RAM.