When Luke's hand gets sliced off, his Lightsaber doesn't extinguish, causing it to at some point slice through the power source for the antigrav generators which hold Cloud City up, which promptly plummets to its doom.
....back on Degobah...
Obi-Wan: *facepalm* "I told you he was our last hope. Now what?!"
Yoda: "Rejoice for those around you who transform into the Force. Mourn them do not. Miss them do not..."
Obi-Wan: "Oh, shut up."