I am using Sonar x3, have a roland quad capture connected to my laptop and keyboard, and am using the quad capture for input and output. One time when I was playing back a recording through headphones connected to the quad capture, the sound suddenly stopped and now I hear no sound when playing back music but sonar is showing sound coming out. Please help.

Is it possible you have sound routed to the headphones output, but are expecting it to come through the main outputs?
 
Did some Preferences values change?
 
Do you also have your audio interface set as the Windows Default Audio Device, as well as having it pointed to by Sonar, while in ASIO Driver Mode?
 
By chance, is the Master fader all the way down, or muted?
 
Bob Bone
 

actually I think that the problem is the quad capture not sonar because another program didn't playback either

So your saying that even after re booting etc the quad capture no longer works? 

The quad capture works as an input - the sound comes through as a waveform, but there is no sound on playback