Can be many things. Lack of fuel, or other electrical problem.
if the engine cranks.... then it is not the battery or alternator.... look at the primary electrical system and fuel injection
If car cranks over and won't start it is not the starter, alternator or battery. Most likely the ignition wires or module, fuel delivery system or air intake meter.
Start engine and measure volts at battery, it should be at least 13
Try jumping it off. If it cranks remove the + side on the battery. If it shuts off the alternator is bad .If it does not shut off clean the battery terminals . Have the battery checked at the Auto Zone.
check your timing belt
First check battery and alternator. I did have it checked.
check your battery or you spark plugs
If it just cranks and cranks and cranks then it's likely fuel pump. If it died and it won't turn over anymore, likely alternator.
As long as the battery is fully charged it will start no matter the condition of the alternator. Eventually the battery will run down if the alternator is not keeping it charged.
You may have a fuel problem, check the fuel filter and fuel pump.
alternator?
your battery could be dead or your alternator first check to make sure that the connections at the battery are tight and free of corrosion if they are corroded the go to a part store and they will have stuff to clean then with. then check the connections at the stater and make sure that they are tight. if that does not work get your car to start and disconnect the negative(black) cable on your battery. if your car dies your alternator is going bad or the connections to the alternator tighten those connections and see if it works if not take to a repiar shop and have them check the alternator and battery