Paradise Lost is an elaborate retelling of the most important – and tragic – incident in the book of Genesis, the first book of the Bible. Genesis narrates the creation of the world and all its inhabitants, including Adam and Eve, the first human beings. Initially, everything was just perfect; God gave Adam and Eve the Garden of Eden to live in, there was no death, no seasons, all the animals were nice, etc. The only thing they were not allowed to do was eat from the Tree of Knowledge. Well, one day the devil disguised himself as a serpent and convinced Eve to eat the fruit from the Tree of Knowledge. She in turn convinced Adam to partake. God became very angry with Adam and Eve for disobeying his one rule; as punishment, he banished them from Paradise, instituted death, suffering, pains while giving birth, and a whole lot more. Adam and Eve effectively "lost" Paradise.
Milton wrote the bulk of Paradise Lost after the restoration of the English monarchy in 1660; he very nearly lost his life. (Milton had written a pamphlet justifying regicide – the killing of the king – when Charles II assumed the throne. Milton – and many others who were not so fortunate – saw his hopes for an alternative English government ultimately end in defeat. For more on the Restoration, head over to "Best of the Web.") While Milton didn't perhaps see this alternative as a "paradise," and while he doesn't allegorize his political life in the poem in this way, the primal story of human loss and devastation must have appealed to the dejected John Milton.