dumb
He was struck dumb by the severity of the punishment.
His next words struck them all dumb.
They were all not only struck dumb with confusion, but were momentarily scared.
I was struck completely dumb when I met my hero at a rehearsal.

SMART Vocabulary: related words and phrases

Falling silent and not speaking
be (on) non-speakersidiom
bite
bite something back
bite your tongueidiom
button your lipidiom
clam
clam up
draw
mouth
quiet
radio silence
silence is goldenidiom
speak
speechlessly
speechlessness
stick
stick in someone's throatidiom
tight-lipped
tongue-tied
wordless