If you lost the passport or found that your passport was stolen during your foreign trip, you may take the following steps.

1. Before calling the authorities and then reporting your missing passport, you should double-check to see if your passport was actually stolen or misplaced. Once the passport has been reported lost/stolen, the passport will be canceled immediately and its details will be sent to the Interpol’s Stolen and Lost Travel Documents database. As such, you will be denied entry or boarding without the valid passport unless you replace it with the new one.
2. If you are overseas, you must contact the nearest U.S. embassy or consulate as soon as possible to get an emergency passport. Of course, you should make sure that you are ready to pay for a fee and you have your photo and ID readily available at hand. Form DS-64  needed to be submitted for the lost or stolen passport. If you have a copy of the missing passport, you should bring it with you for the application for an emergency passport. Notice that this emergency passport is good only for a year. Once you return home, you can exchange it for a regular one.
3. Best time for passport renewal is November ans the worst time for its renewal is March based on the number of passport applications in the recent past.
4. Be aware that some countries such as France may not allow you to enter those countries with an emergency passport.
5. Be aware that even with a valid passport, you can be barred from entering some countries such as Switzerland that requires travel documents (passport) to be valid for at least three months after your planned departure.

For more details, please refer to https://travel.state.gov/content/passports/en/passports/lost-stolen.html