If you are a Lawful permanent resident or Conditional resident and stayed outside the US for more than one year, or beyond the validity period of a re-entry permit, you should get a new immigrant visa to enter the US. If you are a green card holder and stayed outside the US due to circumstances beyond your control, you can get a Returning resident special immigrant visa (SB-1).
If you are not able to return to the US within the one year traveling validity period of the green card or the two years validity of a Re-entry Permit, you can still qualify for a return visa. You are required to apply at the nearest US Embassy or Consulate for an immigrant visa. Once your application for return visa is approved, there is no need for an immigrant visa petition to be filed on your behalf. You have to attend an interview for both your application for returning resident status, and then for the immigrant visa. You have to establish eligibility for an immigrant visa and have a medical examination. You have to pay both visa processing fees and medical fees.
If you are the spouse or child of a member of the US Armed Forces or of a civilian employee of the US Government stationed abroad on official orders, you can use your green card to enter the US even if your green card expired. In such instances, you do not require a return visa (SB-1) as long as you did not abandon your LPR status. To be eligible for a return visa, you will have to prove that you were in lawful permanent resident status at the time of departure from the US and left the US with the intention of returning and have not abandoned this intention. In addition you have to prove that you are returning to the US from a temporary visit abroad and, if the stay abroad was protracted, this was caused by unavoidable circumstances beyond your control for which you were not responsible.
You should submit a completed Application to Determine Returning Resident Status, Form DS-117, along with your permanent resident card and re-entry permit (if available) to the US Embassy or Consulate where you will apply. You must also submit other supporting documents. A Consular Officer will review your application and documents to check if you meet the requirements for SB-1 status. If you satisfy the requirement, you must qualify for the immigrant visa in all other respects in order to get a SB-1 visa.
The US Embassy or Consulate will give you all specific instructions regarding further processing for your SB-1 visa and it differs from Embassy or Consulate. After initial processing, the consular officer decides that you do not satisfy the requirements for a SB-1 visa on the grounds that you have abandoned your residence in the US, it may or may not be possible to get a non immigrant visa depending on whether you have established a residence abroad to which you will return. If you cannot submit sufficient and convincing evidence of compelling ties abroad, you will then have to apply for an immigrant visa on the same basis and under the same classification by which you immigrated before.