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.

(NOTE: Posts should add additional instructions on applying for a returning resident visa here.  Application instructions vary from post to post).