The speaker's guilty thoughts start with consideration of his blindness. You get the feeling that he has had these thoughts before, and maybe even often. Does he blame himself for his lack of "light"?
Ere half my days (line 2)
How does he know how many days he has left? Isn't it presumptuous of him to say how long he "should" live? If you're a Christian like Milton, then God decides how long you live. Clearly, he is looking for some kind of meaning in or justification of his blindness.
and present My true account, lest he returning chide; (lines 5-6)
According to the Christian religion, on the Day of Judgment, or at the end of his life, the speaker will have to relate how he has spent the time that was given to him on earth. He worries that God will "chide" or blame him for using the "currency" of his talents unwisely.
"Doth God exact day-labour, light denied?" (line 7)
The sonnet presents a back-and-forth of guilt and blame. The speaker feels guilty about his uselessness, but he also very subtly blames God for being so demanding. At least it seems so to him.
I fondly ask. (line 8)
And there's the guilt again. Immediately after asking the question, the speaker admits that it is "fond," or foolish. Do you think that the speaker is reflecting on thoughts he has had in the past, or is the sonnet meant to be a blow-by-blow account of his present thinking?