As Poe writes himself in his Philosophy of Composition, an essay about competent poetic writing based on his own analysis of The Raven (full text available here):
I had now gone so far as the conception of a Raven, the bird of ill-omen, monotonously repeating the one word "Nevermore" at the conclusion of each stanza in a poem of melancholy tone, and in length about one hundred lines. Now, never losing sight of the object- supremeness or perfection at all points, I asked myself- "Of all melancholy topics what, according to the universal understanding of mankind, is the most melancholy?" Death, was the obvious reply. "And when," I said, "is this most melancholy of topics most poetical?" From what I have already explained at some length the answer here also is obvious- "When it most closely allies itself to Beauty: the death then of a beautiful woman is unquestionably the most poetical topic in the world, and equally is it beyond doubt that the lips best suited for such topic are those of a bereaved lover."
It should be noted, however, that the authenticity of "The Philosophy of Composition" has been called into question, with one biographer describing it as "a rather highly ingenious exercise in the art of rationalization than literary criticism." (Hat-tip to BESW for informing me of this.) So, although other critics do believe "The Philosophy of Composition" is what it pretends to be, we must acknowledge that it's not necessarily reliable as a source, and look additionally to the poem itself.
From my books surcease of sorrow—sorrow for the lost Lenore—
For the rare and radiant maiden whom the angels name Lenore—
Nameless here for evermore.
It's only the angels that speak her name now, and "here" (in the mortal world?) she is nameless for evermore. This suggests she's with the angels, i.e. dead.
Ghastly grim and ancient Raven wandering from the Nightly shore—
Tell me what thy lordly name is on the Night's Plutonian shore!"
The speaker believes the raven has come from Pluto, and asks it about Lenore. This suggests she too is there, in the "Plutonian" land of the dead.
Tell this soul with sorrow laden if, within the distant Aidenn,
It shall clasp a sainted maiden whom the angels name Lenore—
Clasp a rare and radiant maiden whom the angels name Lenore."
He speaks of seeing Lenore again only in Aidenn, or Paradise.
None of these quotes quite confirms that Lenore is dead - she could be gone and yet the speaker hopes to see her again when both are dead - but it's at least strongly suggestive. Together with the Philosophy of Composition, I'd even venture to say it's conclusive.