If only one Bud isn't charging, the first thing you'll want to do is check for debris in the AirPods case. Use a Q-Tip or another soft tool to gently cling the prongs in the AirPod slot to ensure your bud is getting a good connection. If your AirPod case isn't charging, check the charging port for any debris. You can use a flashlight to inspect the port, then gently use tweezers or a Q-Tip to remove anything blocking the prongs. If you've tried removing debris and they still aren't charging, you can contact Apple. The company sells replacement buds and charging cases for those who don't have warranty coverage.