"Then it all crashes down and you break your crown.."
It all goes downhill for King Lear because he gives up his kingdom to his evil daughters (Regan and Goneril) and loses everything, including his power, money, and his favoured daughter (Cordelia).
"And you point your finger, but there's no one around"
King Lear blames his evil daughters for his demise, when it should be himself that he should blame. Eventually he would come to realize the folly of his ways, however that realization would come too late.
"But the castle's crumbled and you're left with just a name"
King Lear's reign ends because of his foolishness and his evil daughters deceit. His once mighty grip upon his kingdom is now nullified and he's just left with just a memory of his rule. People still regarded him the king, however he has no money and no power. In Act IV, Scene 6, we see Lear dressed in wild berries and donning a crown made from weeds -- Lear apparently still believes himself to be a King, more realistically, a King of nothing.
"Where's your crown, King Nothing?"
In Act I, Scene 4, Lear asks Oswald, "Who am I, sir?" He responds by saying, "My lady's father." -- Which angers Lear. From this segment, we can clearly see that Lear's authority has clearly disappeared. Oswald answers him with no respect, then again what foolish person deserves any respect for giving away his kingdom in haste?
In the same scene, the fool speaks to Lear, "I am better than thou art now; I am a Fool, thou are nothing." Which is to say the fool has a title, and Lear has none. He has given away his crown and thus becomes nothing. Lear is the King of Nothing.