Hello sougatadutta.
Thank you for using Apple Support Communities. Thanks for sharing these details about your left AirPod disconnecting on you. We also see that you're already coming from a helpful resource below:
If your left or right AirPod isn't working - Apple Support
This resource does go over very helpful troubleshooting steps you can try. Were you able to successfully perform each step listed? If you pair your AirPods to a different device, does the same issue continue to occur or does this issue only occur while using one specific device?
If this issue does occur on multiple devices and the steps within the resource you came from don't help you, please reach out to Apple Support. They might have to evaluate your AirPods to make sure everything is working as it should be. Just in case, you can reach them by using the link below:
Get Support - Apple
We hope this information is useful. Best of luck.