To astonish / amaze / astound — express strong wonder or disbelief
To startle — a sudden shock or fright
To stun / stupefy / flabbergast / dumbfound — to leave someone speechless or motionless
To bewilder / perplex / baffle — to confuse or leave someone disoriented
To catch off guard / blindside / ambush — to come upon someone unexpectedly
To shock / jolt / jar — an abrupt, often unpleasant impact
To awe / overwhelm — inspire reverential wonder
To nonplus — to leave someone at a loss for words or action
To disconcert / unsettle / throw — to disturb composure
To bowl over / knock sideways — informal expressions for being overwhelmed
To take aback — to cause someone to step back in surprise