Milton supported the English Commonwealth under Oliver Cromwell which ended up executing the king. But then the Commonwealth came apart and the Stuart monarchy was restored, putting Milton on the political outs. Many others who had supported the Revolution were executed when the new king came into power, but somehow Milton survived on the margins.

The Hackett Classics edition of Paradise Lost does a good job explaining all of this in the introduction as well as the footnotes: https://www.amazon.com/Paradise-Lost-Hackett-Classics-Milton...

Paradise Lost is one of the most beautiful poems in the English language. It takes a bit of getting used to before you can read Milton fluently (he structures his sentences in unusual ways sometimes to accommodate the meter) but after a few pages you'll begin to grasp what a monumental achievement it is. It's a shame it's not more widely appreciated, especially as its themes transcend mere retelling of the biblical story and enter on universal human concerns.