It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say.
An engine needs three things to run mechanical integrity fuel and spark. The engine may be OK mechanically, you have fuel but do the injectors put the right amount of fuel into the engine? Does the ignition fire at the time it is supposed to? The computer in your car takes in readings from sensors on the engine (like crank shaft position, camshaft position, engine coolant temperature, air temperature, volume of air being drawn into the engine, ect...) based on these inputs the computer in your car determines how much fuel to squirt into the engine and when to send spark to set fire to the fuel. If any engine sensor is giving an erroneous reading the computer does not make proper calculations and drvability issues occur. In most cases theses sensor values are needed to "turn on" the fuel pump. If the Check Engine/Service Engine Soon light is on there will be a code stored in the memory of the computer of your car that will indicate where the problem is or at least where to start looking. Even if the light is not on a professional grade scan tool attached to a 16 pin diagnostic socket under the dash of your car will be able to look at live sensor data so a mechanic can determine if sensor inputs to the computer in your car are correct and the computer outputs are correctly controlling fuel and spark. Using a scan tool to examine each sensor can eliminate have to to do intrusive testing at each sensor and also checks to a certain degree the integrity of the wiring harness of your car.
Check for spark at the spark plug, check that fuel pressure is present and that the fuel injectors are being triggered and have power.
Kia as a car manufacturer put a lot of great technical information and work shop manuals on line for free for Kia owners. www.kiatechinfo.com