Try un-pairing and reconnecting them first.
If that doesn’t work right away, you can try these steps:
# Play music with your AirPods connected and turn the volume to 0.
# Disconnect the AirPods in Bluetooth settings (but don’t unpair them, ''and leave them in your ears'')
# Start playing music from your phone’s speakers and turn the volume down to 0.
# Reconnect your AirPods, and you should be able to turn the volume up to normal volumes!