Dumb question but I went thru this with my son last week. Is the gear shift all the way in Park? There is a safety switch that keeps you from starting the car in anything but park. Other than that, check the cable connections on the battery.
My son drove my Taurus for the first time and when he came out to leave it would not start. I drove 15 miles to jump start it. I reached in and pulled the gear shift up and it started right away. Embarassed the kid but he learned.