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. The engine needs three things to start, fuel (gasoline), ignition (spark) and mechanical integrity (it must develop compression in each cylinder to start ie mechanical components must be in synchronization with each other. If it had run well previous to this no start condition chances are the engine is well mechanically.If the engine had run rough, driven poorly, made unusual noises, "ran hot", or switched off while driving the problem may actually be mechanical. Had the Check Engine light been on? The engine may have lost spark or fuel which could be due to any one of a number of sensors having failed or the fuel pump or related components are not delivering fuel to the fuel injectors. The engine uses many sensors to report engine temperature, air flow into the engine, crankshaft and crank shaft position sensors to allow the computer in your car to make calculations controlling fuel and spark to the cylinders. It is hard to speculate without being able to hear the engine crank over and do some preliminary tests to determine the cause of your no start condition. Remove the fuel hose that goes to the fuel rail and get a friend to crank over the engine to see you have adequate fuel volume (you should have approximately 1 pint of gasoline in ten seconds of cranking). Off course take proper precautions as gasoline is highly flamable. Next "listen" to the fuel injector. Using a mechanic's stethoscope (or use a long screw driver placing the handle end up to your ear place the metal tip end of the screwdriver against the body of the fuel injector), get a friend to crank over the engine while you "listen" to the fuel injector listen to each injector at the same point of the injector body. You should hear "click, click, click" as the injector is electrically opened and closed by the computer. Next check for spark at he spark plug to see if you have lost ignition.