A permanent resident (called lawful permanent resident or LPR) or conditional resident (CR) who has remained outside the United States for longer than one year, or beyond the validity period of a Re-entry Permit, will require a new immigrant visa to enter the United States and resume permanent residence. You can learn more about Returning Resident Visas on usvisas.state.gov.
Update on Executive Order 13769:
The provisional revocation of January 27, 2017, which had revoked all valid visas of nationals of Iraq, Iran, Libya, Somalia, Sudan, Syria and Yemen in accordance with Executive Order 13769, “Protecting the Nation from Terrorist Attacks by Foreign Nationals,” is now lifted, and those visas are now valid for holders otherwise eligible for their visas. Anyone with a valid visa may now use it to travel to the United States and does not need to apply for a new visa.