I had the exact same problem happen to me today.
My left channel just stopped. It is indeed a problem with the channel and not the speaker itself. When I plug that speaker into the right channel port at the back of the subwoofer, it works fine.
I suspect something has blown inside but haven't opened it up for inspection...I only bought these speakers on July 31 so don't want to risk voiding my warranty or anything.
With a few people reporting similar issues, I wonder if there is a design fault going on?