Originally Posted by shelbyfv

The simplest solution is to replace the derailleur with the same model. If this isn't possible for some reason and you want recommendations for a substitute, you'll need to provide info. Current group including shifters, derailleur, cassette (tooth range) and it won't hurt to say what bike it's on. Basic info. If this is stuff you can't identify, then you shouldn't be messing with it, take it to a shop.