To consternate someone is to worry or alarm them. If your talented pitcher suddenly loses the ability to pitch a single strike, it will consternate the other players on your softball team.
Things that consternate you cause you concern and dismay, sending you into a startled state of confusion. The word comes from the Latin consternat- ("terrified or prostrated"), which derives from another word meaning "throw down." If something is so disorienting and anxiety-provoking that you feel you've been flung to the ground, it consternates you. A pop quiz in your hardest class might consternate you, as might suddenly remembering you have to give a speech you haven't prepared for.