I saw a similar problem with my M-Audio Fast Track Ultra recently.
In my quest to streamline my DAW and make things work better/faster, I tried disabling Windows Sound System completely. I went in to System Configuration and prevented it from loading.
Everything worked except my Midi port on my Fast Track Ultra. For some reason Sonar will not see it unless I have Windows Sound System enabled during boot up. My thinking was, it's a port in a separate USB Midi device and it should work, but I'm obviously wrong. Took me forever to figure out what was going on until I realized I had disabled Windows Sound System a few days prior. I hadn't used the Midi port right away after I did the tweak.
Did you change any settings in Windows Sound System?
Have you in the past and maybe performed a Windows Update that may have changed something?
Just thought I'd post this in case it may trigger a memory of something you changed.
Good luck.
Bub.