I have been using Sonar products, pretty much trouble-free, for years. Now, as of last night, I get no playback.  My interface is a Steinberg UR44. The interface still works for other programs like iTunes. Only in the Sonar program I'm having a problem.  Funny thing is when output from the "main output" jacks on the interface, I get no sound.  When I output out of the interface "line output 3&4" jacks, I get sound.  Anybody out there with suggestions? I'm lost. 

Hi, and welcome to this forum. It sounds to me like the outputs are in use elsewhere.
Have you checked in Preferences (P), Audio (both Devices and Driver Settings) to see if they are available there? Do you have share drivers with other programs checked in Playback and Recording?
All the best.