How can I fix my Thompson set top box (7 years old)?

When I change the volume with the Thompson settop box remote control, the volume bar appears on my LG TV screen but it will not disappear once I have changed the volume. The only way I can get the volume bar to disappear is by turning off the TV and settop box and then turning them back on or by pressing the OK button on the TV remote control.

I want the volume bar to disappear by itself, as it did until the last few days. Can anybody assist?

Thanks

Mark
January 2012