If you happened to lose your Certificate of Naturalization, you need to file a Form N-565, an Application for Replacement Naturalization/Citizenship Document, to USCIS and you will get a new Certificate of Naturalization. You can get an N-565 by calling the USCIS Forms Line (1-800-870-3673) or downloading the form online. You need to submit this along with the fee to your local USCIS office. It usually takes up to one year to get a new certificate.
Meanwhile, you can use your passport to prove your citizenship. They have recently replaced Alien Registration Cards to Permanent Resident Cards. However, this doesn’t mean that you have to have USCIS issue you a new version of the card. You need to renew your card only when it expires.
For instructions on Renewing or Replacing a Green Card, Permanent Resident Card or Alien Registration Card visit: Green Card Renewal & Replacement site.