The poem opens with the word "When," and a quick survey of the poem reveals that sentence doesn't end for a long time—indeed, the only period in the poem comes at the end of line 14. The poem is an extended, single sentence, which can be divided in two: a conditional clause and a main clause. The conditional clause lists a series of circumstances and the main clause then explains what happens in those circumstances. The word "When" in the first line of the poem introduces the conditional clause, and the next four lines reveal what that "when" consists of. In other words, these four lines describe a situation: a situation of considerable despair and despondency for the speaker of the poem.
The speaker begins by declaring that he is doubly in disgrace—both fortune and other people have turned against him. The speaker uses synecdoche to bring those other people (and their judgment) into the poem. When he mentions "men's eyes" in line one, he doesn't mean (or doesn't only mean) that people are looking at him askance: the eyes stand in for the fact that people are judging him. Just as the eyes imply that there is some intelligence, some agency, making active decisions about his character and worth, so too the phrase "in disgrace with fortune" suggests that fortune itself is making judgments about him—that fortune has its own intelligence and agency, and thus has its own capacity to affect the speaker's life.
The next 3 lines of the poem register these effects on the speaker's life: he is "alone" and he is an "outcast." He weeps over his condition, and he prays to heaven for relief. But his prayers are "bootless"—that is, useless. They fail to improve his lot, and so he looks at himself and curses the circumstances that brought him to this point.
The lines are highly charged with emotion—when this speaker is not weeping, he is crying out to heaven or cursing his fate. Indeed, they may even be melodramatic. Immediately, then, the reader faces a major interpretative issue: whether to take the speaker seriously. If the reader does, the eventual resolution of the poem is a powerful testament to love's capacity to assuage the wounds of the world. If the reader doesn't, however, the poem becomes melodramatic and unconvincing.