Astounded means very surprised, often so much that you cannot speak for a second. It is the usual -ed adjective from astound: “I was astounded.” You can be astounded by something, or astounded that something is true. In feeling it matches stunned or “I couldn’t believe my eyes.” It is common and natural — more people say “I was astounded” than “It astounded me.” It describes a state after something happens, not a long-term kind of person. The thing that causes the feeling is astounding; the person who feels it is astounded. Same pattern as boring / bored.