Perhaps in this neglected spot is laid Some heart once pregnant with celestial fire; Hands, that the rod of empire might have sway'd, Or wak'd to ecstasy the living lyre.
Now the speaker is reflecting on what type of person might be lying in the unmarked graves in the churchyard
Maybe, in the churchyard, there lies a person whose heart was once full ("pregnant" means full, here) of what the speaker calls "celestial fire."
Huh. What could that mean? Sounds like a metaphor to us, since no one's heart is literally full of fire, celestial or otherwise. "Celestial fire" must be a metaphor for passion.
Maybe, in the churchyard, there lies a person whose hands could have ruled an empire. Or someone whose hands could have played a lyre (a kind of old-school harp) so well that the lyre would have become conscious. That's playing a mean lyre!