Starter dead, or loose connection to it. I had the same problem not too long ago, and it turned out all I had to do was clean the battery terminals [the car and battery terminals saw a full 12.6v but the starter didn't see half that. So it never worked. Easiest way to test this is to grab a multimeter and check the voltage at the starter [a big stud with a nut holding a large cable is where to test at], while someone tries to start the engine. If you don't have a multimeter or want to find out hit or miss if it works, take a jumper cable and only connect one of the cables to the positive terminal [don't hook it up to the negative, it isn't necessary], then hook up the other end of the cable to that starter cable stud, and try to crank the car over. If it does nothing, the starter has most likely failed, or the battery has died.
Oh yeah, make sure the battery is fully charged for this.. that'll help the tests