"On His Blindness" is an autobiographical poem that Milton wrote about losing his sight at a relatively young age. Writing much of his poetry about religion and Christian theology, Milton felt that he was in the service of God. He is frustrated by losing his sight as anyone would be. He is particularly frustrated because he has lost his sight while working in the service of God.
When he discusses losing "light," he refers to the loss of his sight and the potential loss of his talent (his intellect and writing). Referring to a Biblical passage, Matthew 25:14-30, the speaker of this poem notes that it is a sin not to use your God-given ability, "that one talent which is death to hide" (3).
The speaker goes back and forth, from despair to resilience. In lines 1-3, he considers how his light is "spent" meaning used, gone, extinguished. Again, "light" refers to sight and talent. But halfway through line 4, the speaker expresses his resolve to keep serving God. At line 7, the speaker expresses frustration, asking if God expects him to perform a day job (one which is more easily done with light) in darkness.
"Doth God exact day-labour, light-denied?"
I fondly ask. But Patience, to prevent
That murmur, soon replies: "God doth not need
Either man's work or his own gifts: who best
Bear his mild yoke, they serve him best. (7-11)
The speaker's frustration is immediately mitigated by his thought that he should be patient and accept this burden (yoke). He uses the word "murmur" because he is conflicted about whether to despair or persevere. Had he been completely frustrated, his murmur would have been a shout. Had he been completely resilient, he would never have murmured or shouted any complaint. A murmur is like a half-hearted complaint. He is frustrated but resilient. Therefore, his complaint comes out quietly and humbly.