This excellent poem contains a number of different examples of figurative language. I will pick out two examples, but I hope that reading my examples will encourage you to go back and analyse the poem more deeply and try and pick out a few more examples of your own.
Firstly, let us start with the speaker's description of England, which of course necessitates his call for Milton to come back and act as a kind of prophet:
Milton! thou shouldst be living at this hour;
England hath need of thee: she is a fen
Of stagnant waters:
Note the metaphor that compares England to a "fen / Of stagnant waters." Clearly this metaphor is used to capture the sense of stagnation and moral corruption and decay that the speaker feels is afflicting England at this time. The image is particularly appropriate in order to convey this impression.
Secondly, consider how the speaker describes Milton himself and how he lived when he was alive:
Thy soul was like a Star, and dwelt apart;
Thou hadst a voice whose sound was like the sea:
Two similes are used to convey how different Milton was. His soul, we are told, was "like a Star" and therefore it lived apart, distant from the corruption and decay of ordinary man. In addition, his voice spoke "like the sea," clearly indicating the way in which he had the power to change and to speak words of wisdom that were different from what others were saying.