Try un-pairing and reconnecting them first.

If that doesn’t work right away, you can try these steps:

1. Play music with your AirPods connected and turn the volume to 0.
2. Disconnect the AirPods in Bluetooth settings (but don’t unpair them, and leave them in your ears)
3. Start playing music from your phone’s speakers and turn the volume down to 0.
4. Reconnect your AirPods, and you should be able to turn the volume up to normal volumes!