What might cause loss of sound in one speaker?

Today I put on a record and there was no music coming out of my left speaker, only a static hiss sound. When I mute the line stage the hissing goes away. I have a Aesthetix Rhea phono stage, a Calypso line stage, and an Ayre V5-xe amp. I have checked all of the connections and everything seems to be connected properly. Additionally, I gave the tubes in each unit a quick visual exam, but didn't see anything that seemed unusual. However, I know nothing about tubes and am not sure if there would be a visual cue if something was awry. Can anyone give me some suggestions of what might have gone wrong, how to confirm it, and the best solution for that problem.