It appears to be plujgged. I have removed the top cover but that is as a far as I get. Does anyone have a diagram of the valve which will help me take it apart.

Is this your fill valve? (see image) If so you have to replace the washer (see image) So you have a FluidMaster type ball cock. This is a common problem with these ballcocks. They have a small washer with a hole,(see image) in the center that swells shut in time preventing water from entering the tank.
Purchase a FluidMaster Seal#242 from your local big box store. To replace it you will have to shut the water off to the tank and take the tank lid off. Twist the top of the ballcock off counterclockwise, (see image) and pull up. Pry out the seal and replace it. Reassemble, turn the water back on and give it a flush. Better now? Regards, Tom

Yes. It is my valve.