Sliding bathtub door w/ mirror

I have a sliding bath tub door, mirrored on one side, that has lost a portion of the mirrored surface.A miusfigured, 1- ft circle of the reflective surface has just disappeared, leaving a gray, non-reflective, allbeit, equally smooth surface in its place. I am at a loss as to what caused this or what to do about it. Besides replacing the entire bath door assembly, is there any other, less expensive solution out there for my problem? I installed the unit, perhaps, 20 years ago. Thanks.