Last Stanza Ode to a Nightingale

By the last stanza he is 'coming down' from his vision.  He discusses the word forlorn tolling him back to his true self, a play on the imagery of a funeral bell in a small town, a symbol of lament for death.  The "plaintive anthem" is fading; his connection to the bird is winding down.  There is significance that the anthem is fading into nature: 

Past the near meadows, over the still stream,

Up the hill-side; and now 'tis buried deep

 In the next valley-glades:

Perhaps nature could be thought of as the source for the anthem, and in that case it is returning to the source in the same way Keats will eventually return to the source when he is buried in the earth.  Or if there is a distinction between the anthem and nature, the anthem is succumbing to nature, and this interpretation gives nature a sense of power.

It ends with

Was it a vision, or a waking dream?

  Fled is that music: - Do I wake or sleep?

At this point Keats is contemplating the meaning of what he's just experienced.  Maybe he is contemplating the meaning/importance of what he's just experienced, or maybe even if he should indulge in such fancies later on..."Do I wake or sleep" is in the present tense; one could argue that he is deciding how to view consciousness from this point forward having experienced 'the music.'

The last stanza wraps up his experience, and shows him starting to wonder how to interpret this vision.