As others have said, check everything else first.

If it's possible that the cabling has gotten miswired, you might have the left speaker powered by FR and FL, and the right by FR and GND. (ie somehow FR and GND have been transposed.) This would give the appearance of no left signal with a mono track, and very limited signal for most other tracks. Try adjusting the balance - if the left speaker signal appears and gets louder as the balance is moved further away from center then this is a possible culprit.