These two phrases mean the same thing, and emphasize how afraid Zechariah was.
# When Zechariah saw him
"when Zechariah saw the angel." The source of the fear was the frightening nature of the angel. Zechariah had not done anything wrong.
# fear fell on him
Fear is described as if it was something that attacked or overpowered Zechariah. (See: [[rc://en/ta/man/translate/figs-metaphor]])
# Do not be afraid
"Stop being afraid of me" or "You do not need to be afraid of me"
# your prayer has been heard
"God has heard what you asked." The following is implied and could be added: "and will grant it." God did not just hear what Zechariah prayed for; he was also going to do it. (See: [[rc://en/ta/man/translate/figs-explicit]])