It could be any of a few things:
starter
keyswitch
neutral safety switch
poor connection
DEAD battery (but it would need to be really dead to not even click)
try starting it with a screw driver on the starter
did you check your battery and alternator?
Ran out of gas? Or ran out of oil?
TRY TURNING THE KEY OVER.
Does it turn over? No Fuel? No Spark? No compression? Weak or bad battery?
neutral switch
Assuming you have checked that the battery cables are clean and tight on both ends and you do hear a click coming from the starter area, I'm going to say the starter/starter solenoid is bad. If you don't even get a click it may be the neutral safety switch.
my first thought would be to check the starter . if you can tap on the starter and it starts after that..you have a bad starter .
When you try to turn the car over, does it click? If it doesn't click, and the interrior lights stay on, then it is your starter. If it clicks, but just wont turn over, then try getting a tune up.
Among other reasons, a pulley or a belt might be broken/damaged.
turn key over turn key over
Does it turn over? Does it turn over?
put the parking brake on the first click. it wont engage the brake but will turn of the lights but if you have a stock radio it will start beeping at you as soon as you hit about 10mph