Every now and then I like to mention something really stupid I did so others don't have to make the same mistake.
After a hiatus from recording due to a vacation, I returned to recording in SONAR and set up a guitar track from my favorite guitar track template. It sounded exactly as expected. I wanted to practice along with the track, so I hit play and...no guitar. It was as if it had been muted.
Perplexed, I did the usual: close the project and re-open, close SONAR and re-open, re-boot computer. Long story short: Before leaving, I was doing a mix but had lots of input echo buttons on because I was still doing overdubs. The mics were picking up noise and the guitars were picking up hum, so I went to Preferences and under Audio, Playback and Recording, checked "Disable Input Monitoring During Playback" so I wouldn't hear the open input garbage while mixing.
But I forgot to uncheck it afterward, hence the problem when I came back to SONAR and couldn't hear the guitar...input echo was still being disabled. So, add this to your list of "stupid mistakes not to make."