Sure (noting that this is quick and off the cuff, and I'm sure a Milton scholar coming across it would scream; also not that unlike Milton, I am not a superb Christian scholar, so I'm happy to defer to others who are):

The first part is about Milton's talent for writing having been his way of serving God, and going blind (in his early forties) making it harder and harder to continue that service. It's also a non-unsubtle reference to the Parable of the Talents.

“Doth God exact day-labour, light denied?” is Milton asking how he can work and serve god without the light he needs.

The he personifies the virtue of Patience replying to him, and telling him that serving God can be done in many ways, and from the heart, and simply being ready to serve is what's needed.

(Of course, Milton wrote Paradise Lost and Paradise Regained after going blind. But when he was losing his sight and writing this poem, he had no idea what the future would bring.)