There are some common USB issues...not sure if they apply here, but you never know.
 

• USB 3 vs. 2. Some 2.0 interfaces aren't happy with 3.0. Some are happier with 3.0. Who knows?
• USB port sleep being active, which you need to turn off in Device Manager.
• Forgetting to turn on the USB interface, and then Sonar uses the onboard sound card. Now it thinks the onboard sound card is what you want to use because that's what it sees.
• USB having a bad day, going into a bar, getting drunk, and passing out. Once it's passed out, it doesn't work any more until it becomes sober.

 
I did manage to solve a variety of "weird" USB problems by getting a USB 2.0 PCI card and using it exclusively for USB audio devices.
 

Yes Bob I agree that in most cases on board sound chips don't cause a problem. And I would not go as far as the bios until this became the only solution. But I personally have had this issue at least 4 times now and on different computers and with different usb interfaces.  Three of them had PCI cards. Forth was a standard MOBO chip on my laptop. 
 
What happens is Sonar seems to loose touch with your USB interface if you have closed Sonar and used the on board sound for something else. Next time you start Sonar it reverts to the on board sound. In some cases for me it was every time I booted. My interface is the first thing I fire up in a boot up. Interface-Computer-wait-then audio system. 
 
One one computer I had to physically remove the Card to solve the issue of Sonar not grabbing my interface. It was a sound blaster pci card. I wiped all traces of the creative asio driver and forever Sonar would say the Creative ASIO driver is not available on starting. 
 
Anyways I see our OP is still sleeping or has gone to cut the grass... 

I should have been more specific in my earlier post.
 
I was referring to situations where the on-board sound isn't using a set of ASIO drivers, and Sonar or your other music production software are the only applications using ASIO driver mode.
 
When there are multiple ASIO drivers present that are loaded into Windows memory, they too will be seen by Sonar, when Sonar loads and Sonar is set to a Driver Mode of ASIO.  This CAN be problematic, as folks often find out when they have ASIO4ALL installed along with their more high-end 'real' ASIO drivers from an audio interface.
 
Sooooo, for computers where the Windows Default Audio Device is set to the on-board sound, AND that set of drivers is NOT ASIO, if Sonar is set to Driver Mode of ASIO and the audio interface drivers are installed, THEN the on-board sound can exist peacefully to be used by all non-Sonar applications, while Sonar will just as cheerfully use the ASIO drivers of the audio interface.
 
THAT is a better explanation (I hope), of what I meant to describe.
 
I seem to recall that Sonar tended to pick and automatically assign the ALPHABETICALLY first detected audio device when multiple ASIO drivers were 'seen' when Sonar started.  This was often problematic.
 
Folks plugging in their audio interface to different USB ports from one start up to the next also frequently cause themselves problems, as many times the installed drivers will not be 'seen' by Windows or Sonar.
 
MAYBE, the original poster will come back from hiatus to engage in their thread.
 
I will consider my above posts to be all I will put out there for this thread, until the OP comes back into the discussion, so until that happens I will bow out of this discussion.  Also, I feel for you on those other bizarre problems from having a second set of ASIO drivers present in the computer.  Glad you have that squared away enough to get your work done without going any more crazy then you currently may be.  :)
 
Bob Bone
 

Yep It's a common occurrence for me to.
The easiest workaround for me is to uninstall and reinstall QC's drivers.
A pain in the but I'm up and running again quickly.  

BENT
Yep It's a common occurrence for me to.The easiest workaround for me is to uninstall and reinstall QC's drivers.A pain in the but I'm up and running again quickly.  

 
That shouldn't be necessary...there must be something else going on.