The speaker in Thomas Gray's "An Ellegy Written in a Country Churchyard" writes of things that will no longer occur for the dead in the churchyard, as well as things the dead will no longer get to do.
The speaker writes:
For them no more the blazing hearth shall burn,
Or busy housewife ply her evening care;
No children run to lisp their sire's return,
Or climb his knees the envied kiss to share.
The fireplace will no longer burn for the dead, housewives will no longer take care of them in the evenings, and children will no longer run to greet them or climb on their knees to give them kisses.
In the next stanza the speaker mentions things the dead will no longer get to do: harvest crops, plow, drive their teams, and chop wood.
The stanzas following these two deal with a different issue. They make the point that no matter what someone possesses--power, beauty, wealth, glory, etc.--that someone will still die. The emphasis here, though, is not what is left behind, but on the fact that death will come no matter what.
Gray does not actually come out and say "here's what's gone forever when you die" but I think that you can determine what he means even so. To me, Gray is saying that the main thing that is gone forever (other than life) is all earthly glory and importance.
My evidence for this is the part of the poem that says "The paths of glory lead but to the grave." If you look at the stanza that contains that line, and the next two, they are saying that all the earthly power and glory that a person has cannot bring them back to life.
So what's gone is life, yes, but also any sort of glory. That means that the people in the country churchyard are just as good as the famous people.
Gray’s "Elegy" is one of the major lyrics of the eighteenth century, and one of the representative poems of the "graveyard school" of poetry, a major theme of which was the need for living a sensible, good life in view of the inevitability of death.
The people buried in the church graveyard are humble, rural folk. Yet the speaker asserts that they are not contemptible because of their simplicity; instead he emphasizes their "useful toil" and "homely joys," pointing out that death is the great leveler, and that "the paths of glory lead but to the grave" (one of the more famous lines of English literature). Some of those buried here might have made great rulers, musicians, defenders of human rights, or poets. But the speaker balances the missed opportunities for good that are buried here by pointing out that the people never had the chance to do evil either. In short, the churchyard is the occasion of reflection on the need for goodness and piety, and the inevitability of death is cause for people to live their lives to their fullest potential.
Gone forever are the missed opportunities to do great things, but, on the other hand, gone forever are the missed opportunities to do evil. Death is the great leveler of life.