dumbfounded

(verb) to be completely amazed and surprised

Example Sentences:

When the children went to see the magician, they were dumbfounded when he made a rabbit disappear.

Mario was dumbfounded when his team lost the championship game.

From the Blogs:

Harford and McCall left dumpfounded by referee’s incompetence

Media:

Similar idioms:

1. flabbergasted
2. gobsmacked
3. fired up