The most common reasons a car does not start, even though the engine turns over, are problems with spark, fuel or compression. An inexpensive spark tester, available at most auto supply stores, is an easy way to test for spark. When held next to the plug wire, a neon lamp flashes if the engine has spark. A weak or nonexistent spark calls for methodical testing to diagnose the problem.
Assuming there is fuel in the tank, the next problem to check is the fuel pump. When the operator inserts the key into the ignition and turns it to the on position, that person should hear it run to build fuel pressure before it turns off. If there is no sound, the fuel pump is likely bad, a common problem with modern vehicles, according to CarParts.com. Another reason the vehicle does not start is improper fuel pressure. A fuel pressure gauge allows the owner to check the system pressure and compare it to the manufacturer's recommendation.
The third likely cause of a problem is a lack of compression. A mechanic's compression gauge screws into the hole for a spark plug. Testing requires removal of each plug and inserting the compression gauge into each hole. Low compression on all cylinders indicates a failed timing belt or chain, which requires replacement before the car will start again.