Car Won't Start?

Your engine needs five things to start and run properly:
  
1) A battery with enough power to turn the engine over fast enough to start.
2) A good starter strong enough to turn the engine over fast enough to start.
3) A engine in good mechanical shape with the correct valve timing and compression.
4) Proper fuel air mixture delivered to engine by the carburetor or fuel injection system.
5) Proper spark of high enough voltage across good spark plugs. This spark must happen at the correct time.
  
Cars that won't start usually have either a fuel supply problem or a no spark problem. A broken timing belt or chain can also stop your car dead! Here are some easy tests to do:
  
Fuel Test
Note: this applies to fuel injected gasoline engines. Almost all engines made after 1990 are fuel injected.

Get a friend to sit in the driver's seat. Go to the gas filler cap. Remove the cap and listen to the open gas filler pipe. Have the friend turn the ignition switch on, but don't turn the engine over (the rrrrrr sound). You should hear a whirring sound from the gas tank for a few seconds, then it should stop. This is the electric fuel pump running. No whirring sound, no running fuel pump. This means your fuel pump is either bad, or the relay powering it is bad, or the computer which runs the relay is bad.
  
Spark Test 
Remove a spark plug wire. Insert a screwdriver or other metal object in the plug wire end. Hold this wire about 1/8 to 1/4 inch from a metal part of the engine (be careful of electric shocks). Have the friend turn the motor over. (making the rrrrr sound). There should be a nice blue spark jumping off the screwdriver to the engine metal part (pick a part, any part). Warning: Do not let the spark get near the battery!

No spark can also be caused by a bad coil, rotor, distributor cap, electronic ignition module, pickup coil assembly, or wiring defect.
  
Timing Belt Test for most cars
First, remove oil filler cap. You should be able to see the camshaft. Have a friend crank the engine. If there's no camshaft turning, then your timing belt is bad. If you can't see the camshaft, remove the distributor cap. With very few exceptions, the distributor is driven off the camshaft. If the distributor doesn't turn with the motor, the belt is probably bad.