Basically the song is about a man [Enrique in this case], who still loves a girl after they have broken up, and he wants her back, but she wants nothing to do with him. 'The lock on the door has changed' means that he no longer has the key to her heart; she has no more feelings for him.
I'm going through this same problem right now.
Basically the song is about a man [Enrique in this case], who still loves a girl after they have broken up, and he wants her back, but she wants nothing to do with him. 'The lock on the door has changed' means that he no longer has the key to her heart; she has no more feelings for him.