Dark patch growing in LCD screen

[Ender]

I have a HP42S that I seldom use. It came to my notice that that there's a dark patch in the middle of the LCD that gets bigger with time. Presently, the dark patch which is elliptical in shape nearly covers the entire screen save the two ends. Other than that the calculator is working fine. Did anyone have the same experience what what did you do to resolve the problem? Thanks.