What John described happened to me several vastly intermittent times in the past, and once I finally learned to check for any hidden dialog boxes (waiting for some response from me), by minimizing the main windows, I was able to respond to the dialog box and keep on running my apps with no further trouble.
Again, this is just supportive comments on the tip - and that too is not just related to Sonar, but to other software as well.
Anyways, glad that you got things up and going again for you,
Bob Bone