If your permanent resident card (green card) is lost or stolen, you should file Form I-90, Application to Replace Permanent Resident card with the USCIS to get a replacement.
If you meet all the requirements needed to qualify for citizenship, but your green card was lost/stolen, you have to apply for a replacement for the lost green card before applying for citizenship as it is mandatory to submit your green card while getting naturalized. However, in such instances, you need not wait to receive the new card for filing for naturalization. As soon as you get the I-797 Receipt Notice for having filed Form I-90, you can proceed further with filing Form N-400, Application for Naturalization.
After you submit the I-90 application package with the submission fees and supporting documents, they will acknowledge receiving the package by sending an application receipt notice with a receipt number. As stated above, you can start filing the Naturalization application as soon as you receive the Receipt Notice for the Form I-90. You are required to include a copy of the Receipt Notice with the N-400 application.
The Application Process
After you complete and mail your application with the supporting documents and submission fees, the USCIS will acknowledge receiving the package by sending you a Receipt Notice through Form I-797. With this receipt notice, you can make an “Info Pass” appointment with your local USCIS office to get an I-551 stamp on your passport. This stamp serves as a temporary proof that you are a lawful permanent resident (green card holder) of the US. You can use this for all practical purposes that you would normally do with your original green card (employment, travel, and evidence of status). This stamp is valid for one year from the date it is issued.
Expired Green Card
A green card comes with a validity of ten years and has to be renewed on its expiration. Even if your green card expired, the replacement process is the same where you have to file Form I-90. It is always safe to apply for your new green card well before you take any trip abroad. Ensure that you bring with you on any trip the temporary evidence for having applied for a replacement you received from the USCIS. If you are outside the US and try to reenter the US with an expired green card, you may face unnecessary problems that will cause long delay during the inspection process at the port-of-entry. It is always advised you travel after you file the I-90 application.
Even US employers will not accept an expired Green Card while verifying employment authorization for new recruits. Per immigration laws, if your green card will expire within six months, you have to renew your green card before filing the application for naturalization. It is mandatory that you submit a copy your green card when you apply to become a citizen.