Wrecked, solitary, here– (line 16)
The combination of "wrecked" and "solitary" makes us think of shipwreck: Robinson Crusoe and Jack Sparrow and all that. When the funeral ends, the speaker is left alone with Silence. We don't know where they are – the speaker only says, "here." Could she be in the afterlife at this point? Is she inside the coffin?
And then a Plank in Reason, broke, And I dropped down, and down– (lines 17-18)
One of the large wood planks in the floor breaks. If you think the speaker is inside the coffin, then this would be like falling to her death after she had already died. If the speaker isn't in the coffin, it would obviously be very ironic to die right after a funeral. There's also a much more conventional explanation of these lines: she could be in the coffin as it's being lowered or "dropped" into the grave. But we don't think this explanation is any more powerful than the others.
And Finished knowing–then– (line 20)
The speaker's fall doesn't end with a violent death. She "finished knowing" about the experience. This phrase could be a euphemism – a polite expression – for death, but it doesn't sound as traumatic as what came before. Critics who think the poem is about forgetting about a painful memory often point to this line as the moment when she hits the "erase" button, so to speak.