What does it mean to be flabbergasted?

To flabbergast is to overwhelm with shock or astonishment; dumbfound.

"Hey, Alexa!" a male voice called out.
Alexa turned abruptly, her smooth brown hair swinging over her shoulder, searching the crowd for the source of the voice. When she realized who it was, she was absolutely flabbergasted. It was Doug, the popular start of the varsity lacrosse team. The same Doug who rode in her car pool every day and yet still managed to pretend that she didn't exist. Alexa stood there, motionless, too shocked to speak.
From Karen B. Chapman's The Mayan Mission.