My husband has been restoring antique furniture for some 35 years.  When he runs across a "sick" mirror in dire need of resilvering, he tells his customer they have 2 choices. (1) Have the mirror resilvered by a professional silversmith, witch can be rather expensive depending the size of the mirror.  (2) Lay the mirror face down on some card board and coat the old silver with Muriatic Acid,  The cover it with news paper and soak the paper with Muriatic Acid. ( careful of the fumes, they may choke you)  After an hour scrub the silver off with an old broom recoat if necessary, until all the old silver is gone.  Wash with warm soapy water, rinse with a garden hose, dry the beveled glass.  Take the measurement or a pattern to a glass shop, and have them cut a thin mirror to match the size of the old mirror.  When remounting the mirror in its frame, put the thin mirror behind the old mirror and you will have the effect of a resilvered mirror.