An assignment asking you to paraphrase a poem is designed to encourage you to read a poem closely and put it in your own words. The first step in paraphrasing this poem is to work out how words are being used by Milton in ways that might different from modern usage. In the first line, the phrase "light is spent" means a light that has gone out or become extinguished, as in an oil lamp when all the oil is used up.
From the title of the poem and the biographical information we have about Milton, we know that this should be interpreted as referring to Milton's blindness, which came upon him in middle age, which is the meaning of the phrase "ere half my days".
The one "Talent" has a double meaning referring both to the Biblical parable of the servant who hides the one talent (a bar of precious metal) rather than investing it and Milton's own talent of reading and writing which, to a great degree depends on his eyesight. He next asks whether God will blame him if he is unable to exercise his abilities due to his blindness, but concludes that God will appreciate his willingness to do God's work even if his abilities are diminished.
To paraphrase this in prose, you may need several sentences to explain every line of the poem, as Milton's language is very dense and allusive when compared with most modern prose.