If you are overseas and you have lost your passport then you need to contact the closest Embassy or consulate. The website detailing the locations is listed in the resource section below.
If you are in the US and you have misplaced your passport and just cant find it then you should report it by calling the state department. There is a central toll free number. 1-877-487-2778. The passport must be valid and not expired at the time you call, once it has been reported as lost or stolen then it cannot be used for travel, so if you find it later it cannot be used.
If you don't want to phone the state department you can mail them a DS-64 form that is a statement confirming the status of your passport. The form and address for mailing can be found on the US Department of state website. The link is in the resource section below.
To get a replacement you are going to need to apply using the form DS-11. You are going to have to take this form in person to your nearest passport agency. Locations and details of the form are on the Department of state website below.
