Sometimes the check valve just gets caught sideways. Pull the screen and poke the little plastic rod you see. If the O ring hasn't moved out of the groove, it will, often just pop back in place. If that doesn't work, you can pull the valve out and reset the O ring in the groove and put it back together. It's just a little, plastic, spring loaded, one way valve. A plug will get you by in the mean time. Even when the valve works, as mentioned, a plug is a good idea to keep dirt out.