Single AirPod Not connecting

Did you already try the troubleshooting steps mentioned in this related article?

If your left or right AirPod isn't working - Apple Support

Also make sure, that the latest firmware version has been installed:

About firmware updates for AirPods - Apple Support

In that case, this looks like a hardware problem. You might have to replace the left one.

AirPods and AirPods Case Replacement - Official Apple Support

Also check, if your AirPod Pro are part of this Service Program:

AirPods Pro Service Program for Sound Issues - Apple Support