Yeah, like Cory said...if you have water in it, try to tighten it.
If you don't have it full of water, take it out, clean up the surfaces and make sure the gasket isn't damaged. The gasket does NOT go on the side the nut is on....regardless of whether the nut is on the inside or outside. I've had some before where the gasket is damaged a bit and I was able to put silicone grease and put a thin layer on the gasket. It worked fine.