Her eyes were open, but she still beheld, Now wide awake, the vision of her sleep: There was a painful change, that nigh expell'd The blisses of her dream so pure and deep At which fair Madeline began to weep, And moan forth witless words with many a sigh; While still her gaze on Porphyro would keep; Who knelt, with joined hands and piteous eye, Fearing to move or speak, she look'd so dreamingly.
Ever gotten woken up halfway through a really great dream and been less-than-pleased about it? Madeline's with you. She's been yanked out of her wakeful swoon, and it's a "painful change."
That said, it's not clear whether or not she's awake or still half-dreaming here: Madeline still sees before her "the vision of her sleep."
So, we've got two obvious possibilities here. One is that Madeline's still kind of out of it and semi-hallucinating whatever she's been dreaming about for the last several stanzas. Option two: she's actually awake here, but because she was dreaming about Porphyro she still sees "the vision of her sleep" because, you know, he's right there.
Whether or not she's awake, she's still definitely disoriented, and Porphyro's afraid of moving a single muscle.
Remember how Porphyro begged Angela to help him get to Madeline's bedroom so he could look at her and "see her beauty unespied" (166)? Well, at this point Madeline's doing all of the looking: she's got her gaze locked on Porphyro, and those last words—that "she look'd so dreamingly"—can be read both as "Madeline appeared to be all dreamy" and "Madeline looked at Porphyro with dreamy powers," where "dreamingly" describes the action of Madeline's looking.