Your passport is an important identification and travel document. You need this document to re-enter the US and to travel. If you have lost your passport, it is natural to panic, especially if you are traveling outside of the US and will need your passport to return home. If you are overseas when you lose your passport, immediately contact the closest US consulate or embassy. The authorities at a US consulate or embassy can ensure that you have the documents you need to return home.
If you are in the US when you lose your passport, report your passport lost. You can do so by contacting the US Department of State at 1-877-487-2778 during business hours. You can also file Form DS-64 (Statement Regarding a Lost or Stolen Passport) to report your passport lost. Once you have submitted a declaration that your passport has been lost or have reported the loss to the department of state, your passport is invalidated. This means that if you find your passport after declaring it lost, you will not be able to use that old passport. If you do find your passport, you will need to return it to the US Department of State, where it will be destroyed (unless you specifically request that the invalidated passport be returned to you).
A passport cannot be validated again once it is declared lost. For this reason, you will want to apply for a new passport soon after declaring your passport lost. You can apply for a new US passport by filing Form DS-11 (Application for a U.S. Passport) and Form DS-64 (Statement Regarding a Lost or Stolen Passport) in person at a passport agency or accepted facility.
It is important to declare your passport lost as soon as your notice the loss. Do not ignore a lost passport; if you passport falls into the wrong hands and has not been invalidated it could be used for criminal activity or for identity theft. Declaring your passport lost protects you and paves the way for a new passport.