Looking at old photographs sets off something physical in the speaker. The memories rise "like smoke from a furnace," travelling from the chest, through the throat, and settling behind the eyes, so the reader feels grief arriving in the body before it becomes a thought. The furnace image suggests something that has been burning quietly all along.
The second half is one long, unfinished sentence. It describes searching for feelings we'd rather drown, dreams that crawl into the dark where "creatures hide," and words that lose their meaning until only vague feelings and images are left. The poem seems to be asking what remains when language gives out. It never answers. The closing question, "What happens then?", simply hangs there.
The mood is hushed, uneasy, and a little haunted. The loose punctuation and broken line breaks make it feel like a thought that drifts and trails off, much as memory does.