Could be anything! Are you getting fuel? Are you getting spark? Is a belt broke? Is it cold out? I could continue but I wont, you should consider signing up for an account so you can edit your question with a little more info.
Does it turn over? Does it have fuel, spark, compression?
Is it getting fuel? Does it have compression?
Two common reasons why an engine will crank over, but not start: There is no spark. There is no gasoline.
You may have a weak battery, try jump starting it with another car or battery.
it doesnt start when its foggy out because all the wetness in the air
your battery is dead
(7 Ford Mustang wont start anti theft alarm wont turn off
Does it turn over? Does it turn over?
ingnition switch locks up on 2003 Ford Mustang and only way car will start is if you can turn steering wheel back and forth to get ignition key to turn
Bad ground Bad switch in steering column
Could be the relay. Start there first- cheaper. Then Id check the fuel pump
Will turn over but will not start