When you're dumbstruck, you're so amazed or shocked that you can't say a word. You might be dumbstruck with fear when you hear a scary noise upstairs in your supposedly empty house.
You might be dumbstruck by the changes in a town you haven't seen in several years, or dumbstruck with terror on a roller coaster. The adjective dumbstruck describes someone who's rendered speechless by an overwhelming emotion — in other words, struck dumb, or "silent," which is the oldest meaning of the word, directly from the Old English.