If you are a lawful permanent resident of the U.S., you should have a green card as evidence of your status. In fact, the law requires you to carry your green card with you at all times. And you will want to be able to show it if asked by an employer about your eligibility to work, or for purposes of returning to the United States after travel to another country.
Carrying your green card comes with risk, however. Your green card might be lost, stolen, or otherwise mutilated or destroyed. In any of these circumstances, you will need to apply for a new one.
Another circumstance that might impel you to get a replacement green card is if the information on it is inaccurate -- either because of a USCIS error or because of a change in your own life, most likely a name change.
Report all stolen green cards to the police. Green cards are a hot item, and there is always a possibility that yours will be stolen and sold. If this happens to you, be sure to file a police report. You may not get your card back, but when you apply for a replacement card, the report will help convince USCIS that you didn’t sell your own card.
To obtain a new or replacement green card, the application is made using Form I-90, issued by U.S. Citizenship and Immigration Services. The form is available at your local USCIS office, by mail from 800-870-3676, or on the I-90 page of the USCIS website. You can also submit this form electronically, or through the USCIS website--though you will still have to mail in the supporting documentation, which are as follows:
You will most likely need to pay a fee for this application, which was $450 in late 2016 (but always check the Forms page of the USCIS website for the latest fees). That amount includes an $85 fee for biometrics (fingerprinting, photographing, and the electronic capture of your signature). But if the error causing your need for a new card was made by USCIS, you do NOT need to pay any part of this fee.
Form I-90 comes with a fairly complete set of instructions, and you can read more on the I-90 page of the USCIS website.
If you need to return to the United States from temporary overseas travel, but your green card (or your reentry permit) has been lost, stolen, or destroyed, then your first priority may be to get a document that allows you to board a plane or other carrier. You might not have time to both file the I-90 described above and await a new green card.
In such a case, you will want to fill out USCIS Form I-131A.
You will then pay the required fee ($360 as of late 2016) online and apply in person at a U.S. embassy or consulate. The embassy or consulate should give you what's called "carrier documentation" (a letter or boarding document) within approximately two weeks. Present the document to the airline or other transportation carrier instead of your green card or reentry permit.
You will still need a new green card eventually, however. Follow the instructions above for submitting USCIS Form I-90.