When this happens to me, it's usually Sonar holding onto my audio driver. I could not kill the process through task manager. So I would have to reboot the computer. However, I found there was a much easier, quicker way and that is to reboot the audio interface instead i.e. turn it off wait a bit and then turn it back on again. That will kill the Sonar process that is still running and can't be killed in task manager.

Possibly you guys have had better luck than I have.
But every time I killed Sonar in task manager or every time I unplugged/turned off my interface, hardware device..
The next time I opened Sonar, I received the "No hardware outputs detected".
I would have to go through the whole process of close/restart until my device showed up again.
 
Logging off/logging on was a 2mintue process and I didn't lose any hardware drivers in the process.

It depends greatly on the interface driver. My previous unit, a MOTU 828, absolutely didn't like being turned off. If I forgot to turn it on before booting, or turned it off while the computer was running, I'd have to reboot Windows to have it recognized.
 
My current device, a Focusrite Saffire Pro 40, is perfectly happy being restarted anytime and generally more dependable all around. Both are FireWire devices; I would expect USB interfaces to be better about losing and re-establishing connections.

YES!! Used to happen to me ALL THE TIME. My USB MIDI keyboard was the problem (AKAI MPK49). Pulling out the USB wire and plugging it back in always allowed sonarpdr.exe to end and restart but it was a pain in the ass. So, I now run that keyboard through my audio interface using 5-pin MIDI wires - no USB connection in use for the keyboard. Haven't had the problem since - about a year now.