You go to the nearest US embassy or consulate and file to get a new one.Alternatively, you just need to provide evidence to customs that you are a US citizen, such as a drivers license, SSN, whatever...
It depends in which country are you living. You can easily go to the USA if you are from a country that has an agreement with the USA to let the citizens visit it as a tourist.
Find your nearest embassy, immediately. They can get you a replacement passport within hours, at a fee of course.
http://www.usembassy.gov/
I had to do this once, when in Austria - they are...