Lines 45-52 tell of a life of toil, but the dead here may have once hoped for something better. Your requested lines are cited here, my analysis appears beneath selected lines as necessary:

Perhaps in this neglected spot is laid
Some heart once pregnant with celestial fire;
Hands that the rod of empire might have swayed,

(The speaker ponders a neglected graveyard. The sight leads the speaker to wonder who lies in its silent and untended graves. He wonders if those entombed there had once hoped for something more from life. They may, he muses, had a heart on fire for a more exciting life. One may even have possessed the ability to be a king.)

Or waked to ecstasy the living lyre.
But Knowledge to their eyes her ample page

(Or perhaps one farmer aspired to be a musician, eager to awake to play his lyre; or perhaps he could have been wealthy enough to wake to its beautiful strains.)

But Knowledge to their eyes her ample page
Rich with the spoils of time did ne'er unroll;
Chill Penury repressed their noble rage,
And froze the genial current of the soul.

(Knowledge here is personified as female. She had not allowed the peasants buried here her secrets. Knowledge is the only way out of poverty. As they are denied access, "penury," extreme poverty, ensues. Poverty freezes out any lofty ideals. The "current" has stopped. The poor buried here died without ever having explored their potential.