Very often, if it works fine one day and not the next, then you have to answer these questions for yourself:
1) Anything new installed?
2) Any updates installed?
3) Did you change any settings or preferences in Sonar or your interface control panel?
4) Any USB devices powered off and back on?
5) Did you power off your computer?
6) Did you switch any USB devices to a different port?
It's likely one of these things caused the change.
Regards,
Dan