Thanks for writing. I'm sorry --- but you haven't given me enough information to be able to help. There could be lots of reasons why your cedars are failing, but without knowing what kind you have, what growing conditions they're in, where you live, and other similar information, I can't even make an educated guess.
However, the folks at your county extension office should be able to help better if you can answer their questions. An agent may even be able to come out to your home and look at the trees in person. You can find contact information for your county extension office in the government pages of your local phone book.
---Justin, Senior Garden Editor, BHG.com