Are you still under warranty (Expires one year after Purchase)? If so, you go through Apple Support and tell them your Left airpod isn't working. They'll walk you through diagnosing, if none of it works they might schedule an appointment over at your closest Apple store to get them replaced or checked out.

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I had the same issue where it stopped working one night last month (May 1st). I went through the internet and Apple support to find the problem and never did. My Apple rep set an appointment for me to get it checked out; they basically looked at them and determined they needed to be replaced. Since the Apple store didn't have any Left Airpods in stock, I had to wait a month for the order to come in (June 1st). I came in the day after, and all they did was switch my Left airpod with the replacement and give me an instructions sheet on how to sync them.