You should probably read around or use AdWords for more than a few hours to understand why this is happening. Clicks and impressions are not upadated at the same time. Give it an hour and you'll see impressions.
Steps to how it happened: 1) Someone types in your keyword. 2) Person see's results and then clicks on a link. 3) Person the presses the back button, and goes back to the result page. 4) Person see's your ad, and clicks on it. When that happens, an impressions is not picked up.