If you are astonished, you're feeling the emotional punch of a huge surprise. If you've lived in the city your whole life, the first night you spend in the desert staring at billions of stars will leave you astonished.
Astonished is the adjective form of the verb astonish, which comes from old Anglo-Norman for a blow to the head. We use it now for much more positive feelings, when we are stunned by awe and admiration, and not stunned from being hit in the head with a bat! Synonyms are amazed and astounded.