Most fashion or costume jewelry is made from base metal. It is just like it sounds: a base with a coating applied to give it a particular color.

Typically, the base metal is brass or copper. The coating will eventually rub off or darken. In the case of silver jewelry, it now looks more like tarnished brass. Not good.

There really is no way to restore the silver, except to have it re-coated.  That would mean deconstructing the piece of jewelry, re-coating, and reassembling. For most fashion jewelry this process would cost more than the original piece.

There are some fashion jewelry cleaners that will help, but they do begin to wear down the coating.

Prevention is a better way to go if you want to preserve the shine on plated jewelry. Rather than hang it out exposed to the elements, keep it in a bag specially made to prevent tarnish.  You can find them here at Amazon. 

It won't last forever, but it will maintain the shine on a favorite piece for longer than if left to the elements.

The little dolphin on the left was left to the elements for a year.  The one on the right was stored in an anti-tarnish back for the same amount of time.

Disclaimer: I am not a metalurgist. What I share here is based on either my own research or my own observation.