Undo the three screws, and inside there will be the check valve. It's inline with the fitting and only just under one inch long. Just unscrew it and replace it with a new one. They are very inexpensive, only a few dollars.
The check valve is supposed to seal, and allows pressurized water to go in, but then will seals and will not let any out.
Usually the O ring inside gets out of it's track. They are a pain to fiddle with, and easy to change.
Temporarily, you can put a plug in the line, or a tap on a short hose until you fix it.