Hi, one of my airpod is falling into water yesterday and it does not work? What can I do now?

Here's an Airpods support article mentioning water:

How to clean your AirPods, AirPods Pro, AirPods Max, and EarPods - https://support.apple.com/HT208729 --> section on "Learn about AirPods and water resistance"

In the above document Apple says, "If your AirPods become damaged after they get wet, you can chat or talk with us to order a replacement." Click here --> https://getsupport.apple.com/?caller=kbase&PGF=PGF32003&category_id=SC0285

You can buy a replacement.

AirPods and AirPods Case Replacement - Official Apple Support