Why does my 1999 Mazda miata engine will not start when battery is OK engine turns over and there is plenty of gas?

If the engine turns over, but does not fire up, either no fuel is being sent, or there is no spark to ignite the fuel.

For fuel, it is easy enough to try and start the car. Then remove the spark plugs to see if they are dry. If the car is injecting fuel, you will smell it.

If there is no spark, the coilpack at the rear of the car may be dead. However, that all coils are dead would is rare. More likely, either your camshaft position sensor or crank angle sensors need to be replaced. They monitor rotation of engine parts and tell the ECU when to fire. On a 99, the Camshaft Position sensor is at the top of the engine. See below for related links. It is held by a single 10mm bolt. The Crank Angle sensor is at the bottom of the engine to the right of the main crank pulley.