Can someone explain what happens at the end of the requiem, when does he dies?

Can someone explain this:

"Biff lefts her to her feet and moves out up right with her in his arms. Linda sobs quietly. Bernard and Charley come together and follow them, followed by Happy. Only the music of teh flute is left on the drakening stage as over the house the hard towers of the apartment buildings rise into sharp focus" (112)

Can someone tell me when he died, was this play all a hallunication or was Willy in the past or something???????

Please HELP!! Thanks soo much!