Brian, the chances are that the float mechanism in the fuel tank (which operates a potentiometer to provide the fuel level indication) has either given up or the float is stuck at the bottom and hasn't risen with the fuel level as you've filled up.
But... before you start panicking that you'll have the fuel tank to drain and remove from the car to replace the float mechanism I'd keep using the car and see if it rectifies itself next time you fill up.
The reason I say this is because I had once run particularly low on fuel and filled up on a forecourt with an unusually steep side gradient only to find that the fuel level in the gauge didn't come up and the low fuel warning (the illuminated fuel pump) was still on. I obviously knew I had a full tank of fuel so reset the mileage trip indicator and continued to run it to the normal mileage I was getting from a full tank... about 550 miles in my case for the type of motoring I was doing at the time (but kept a 5 litre can of diesel in the boot just in case
) and the next time I filled up the float came back up and I haven't had the same issue since.
Martin