Adieu! adieu! thy plaintive anthem fades Past the near meadows, over the still stream, (lines 75-76)
When the spell of his vision is broken, the speaker must leave the fragrant, frozen midnight world of the nightingale and return to the real world, where it's just another sunny day. The flight of the nightingale is the height of nature's transience. Just when the speaker was starting to get into a groove, too.
Fled is that music:—do I wake or sleep? (line 80)
Yes, the end of the poem kind of plays that old trick from bad TV and movies – he has been dreaming the whole time! It only takes him two lines to forget all about the reality of the nightingale.