The 1000-year reign of Jesus and the 144,000 are completed by verse 7 of Revelation 20:
7 Now as soon as the thousand years have been ended, Satan will be let loose out of his prison....15 Furthermore, whoever was not found written in the book of life was hurled into the lake of fire.
Revelation 21 shows that the 144,000 return to earth after having served their purpose for going to heaven:
.1 And I saw a new heaven and a new earth; for the former heaven and the former earth had passed away, and the sea is no more. 2 I saw also the holy city, New Jerusalem, coming down out of heaven from God and prepared as a bride adorned for her husband.