" If you have been outside of the United States for less than one year and you have lost your legal permanent resident card/your legal permanent resident card was stolen/your card expired during your time outside the United States, the Immigrant Visa Unit may be able to issue you a Boarding foil to allow you to return to the United States. If you have been outside of the United States for more than one year, please see our information on Returning resident.

In order to apply for a boarding foil, you must visit our embassy with your completed form on any Tuesday afternoon between the hours of 2:30-3:30. You do not need an appointment. In case you were travelling and lost your Green card, you may contact the Embassy between 9:00 am to 10: 00 am with your tickets and application for a boarding foil.

Please bring the following documents/information to your appointment:

Completed Form I-90 (Application to Replace Permanent Resident Card)
A valid passport in good physical condition
Three recent color photographs. The photographs must be 2 inches x 2 inches (5 cm x 5 cm) in size, with no head covering, and showing a full, front-face view against a white background, without earrings or eyeglasses.
Your alien registration number (A number)
Airline ticket specifying date of travel, if you have one
If applicable, a police report documenting the circumstances of how the permanent resident card was stolen
Evidence of your presence in the United States within the last year, if possible.

There is no charge for a transportation letter. The transportation letter is normally valid for one month and for one entry into the United States. Once you arrive in the United States, you will need to take the necessary steps to obtain a new permanent resident card with U.S. Citizenship and Immigration Services. "

http://nepal.usembassy.gov/lpr.html