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.
Try channel-swapping of each component in turn to detect the malfunctioning one.
After swapping cables, swap tubes in line stage output ones first.
Start at the speakers/speaker cables and work backwards. Swap the known-good working speaker/speaker cable with the bad one. If it doesn't work, you've found your culprit.

If it does work, start working back thru the connection chain, swapping in known-good connections/cables until you find the break...

Do you have another source you can try? Most likely culprit is a bad tube but it's possible you lost a channel in your cartridge. At least with a cd player you can rule that in or out pretty quickly.