What is the "thousand year reign?" Who will be reigning? Where will the reign be? When is that reign? Why is there a thousand-year reign? How will it happen? Those are the news reporter's five questions as applied to Revelation 20:1-6. If people would apply those questions to that passage some might be surprised at what they would learn.
The passage really gives no idea of the why or how. We can, however, answer the other questions.
What is the thousand-year reign? It is not, as some contend, Christ reigning a specific thousand years on earth. How do we know?
A thousand in the Bible, unless part of a more specific number, is generally an either an approximation or a hyperbole. Jesus at one time fed "about five thousand" men. (Mk 6:44) and "about four thousand." (Mk 8:9) He later referred to the numbers as "five thousand" and "four thousand." (Mk 8:19-20) Although Jesus used the specific number he was obviously meaning a non-specific approximation of thousands of people. When the psalmist refers to "the cattle upon a thousand hills" (Ps 50:10) as being God's he is not limiting God's ownership to a specific thousand hills. Instead he is using that large number to represent an unspecified area. If the Revelation were a history, like the gospels, we might expect the "thousand" to be relatively specific, an approximation. Most, if not all, people I know consider the Revelation to be symbolic. That is, few would contend that the book literally talks about a woman riding a seven-headed red beast (Rev 17:3) or that Jesus has a literal sword coming out of his mouth (Rev 1:16). If people consider this a book of symbols, how do those same people insist that the phrase "a thousand years" must be one of the few literal things in the book? Why can it not signify a long, indefinite period of time, like David's "thousand hills" represents an indefinite area of land?
Jesus the Messiah is already reigning over a spiritual kingdom. He never intended to rule over this physical earth. "My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence." (John 18:36) He was telling Pilate that he was surrendering his life of his own will because he did not want to be a mere earthly king. By dying he was going to be given an even greater kingdom than could be had on earth. If Jesus is now reigning over an even greater kingdom, why would he take a demotion to reign on earth?
Related to the last thought, who will be doing the reigning for the thousand years? This passage is not limiting Christ to a thousand-year reign. It says absolutely nothing about how long Christ is going to reign. The who of the passage is those who died to the spiritual "world." They will reign the symbolic thousand years, and they will reign with Christ for that time. That limits their reign, not his. Further, the ones reigning with Christ will be those who have experienced the first resurrection. Since all in the church have experienced that resurrection (Romans 6), we must be reigning with Christ right now.
Some may say, "but the ones reigning were dead, and some of the dead were not resurrected." That is true. We are, however, talking in spiritual terms. The first resurrection is for those who died to the world of sin. The second death of God's punishment has no power over them. The "rest of the dead" are those who have never died to sin and will be resurrected eventually to punishment. They will never reign with Christ.
It all looks so simple that one wonders why others can not see. When viewed in purely spiritual terms the thousand-year reign is the life of the church. That will end at the final judgement, when Christ gives up his authority (1 Cor 15:28) and returns the kingdom to God. Until then we should enjoy the chance we have to reign with Christ today.