If your sterling silver has turned to a darker color, it's very likely that it has experienced some oxidation or tarnishing. This is a totally natural process and can even enhance the look of your silver jewelry. Often if you wear a silver jewelry piece all the time, it will stay shiny along with those great depth of detail.
However if you'd like to clean your jewelry, there are certainly ways you can do it!
Quickest and easiest is to use a jewelry polishing cloth. My favorite is the Sunshine Polishing Cloth. These clothes have a micro abrasive embedded in the cloth that brings a bright shine to your silver. Because of the abrasives, it isn't a good choice to use on 14k gold-filled or gold plated/vermeil items.
Another great trick is to use science to clean your silver! Here is what you need:
1) Heat-proof container (I use a piece of Tupperware)
2) Aluminum foil
3) Baking soda
4) Hot water
Place the foil in the bottom of your container and add your silver pieces. I don't recommend using silver with any gemstones or pearls or oxidized detail you want to keep (because this may clean that out!).
Give it a dash of baking soda - a tablespoon or so.
Add hot (does not need to be boiling) water. The mix will foam and you will be able to smell the sulfur (like hard boiled egg yolk). Give it a little shake or stir and the tarnish may vanish almost instantly or it can take a few minutes. For really stubborn tarnish, you may have to repeat the process. Also try a fresh piece of foil after a few turns.
Voila! Very cool, right?
Give it a try and let me know how it goes!