Left AirPod isn't working

Check this link. https://support.apple.com/en-us/HT209581 - If your left or right AirPod isn't working

Maybe, you will need to replace that specific part of your Airpods. You can contact your local Apple Store.

Hi! When you pair them up with your iPhone, is the battery level of your left airpod sufficient enough for operation? If yes; do you see any physical obstruction in the speaker grill of the airpod? Many a time; the build of dust or earwax could affect working though this may not be the case with you but worth checking it out. If all this are done and no visible damage is seen, you may need to take it to your nearest apple care.