If you hold a U.S. Permanent Residence Card (green card), it is possible to lose it if you leave the United States and remain overseas for an extended time. In general, you should obtain a Re-Entry Permit with the US Citizenship and Immigration Service (USCIS) before leaving the US if you plan to remain overseas for more than six months. If your stay overseas exceeds the validity of your Re-Entry Permit (or if you failed to get one before leaving the US), your green card may be revoked. It is possible, however, to apply for reinstatement.
Determine if your stay overseas exceeds permissible limits. If your continuous absence from the US is less than six months, you should have no problem re-entering. If your absence is more than one year, your green card will probably be revoked without a Re-Entry Permit. Otherwise, refer to the expiration date of your Re-Entry Permit--it is typically valid for two years.
Download Form DS-117 (Returning Resident Status SB-1) and Form DS-156 from the US State Department website, and fill them out (see Resources below).
Gather documentary evidence of your "continuous, unbroken ties to the United States." This evidence can include the filing of US tax returns during your absence, ownership of residential property and other assets in the US, evidence of family ties in the US and the maintenance of professional licenses during your stay overseas. The idea is to prove that you originally intended to permanently reside in the US and that you always intended to return.
Gather airline tickets and passport stamps to document the length of your absence form the US.
Gather documentary evidence to prove that your extended stay overseas was caused by events beyond your control, for which you are not responsible (illness, transfer overseas by a US company or the need to be employed in your home country for a certain period of time in order to qualify for retirement benefits, for example).
Have four passport-sized photos taken.
Bring all of the foregoing documentation and apply in person to the US Embassy or consulate nearest to you no later than three months before your intended date of return to the United States.
Enter the United States using your green card and your home country passport bearing your SB-1 visa. Only at this point will your green card be considered reinstated.