"Vehicle Won't Start"
Anytime a vehicle won't start can be a pretty big headache and never any fun, below are suggestion as to why it might not be starting.
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"Vehicle Does Nothing when I Turn The Key"

Dead Battery
A dead battery will cause the vehicle not to start, before you go pulling the battery try turning on your head lights, if they come on it might not be the battery, simply put a multi meter to the battery terminals and test for voltage. You'll want to see upwards of 12+volts,you can also purchase a battery load tester when will put stress on the batter to make sure all the cells are functioning properly, something a battery can test good on a multi meter but still have a weak cell. Any local car mechanic should have a load tester on hand.
Alternator
Even though a alternator isn't what causes your starter to crank, it could be the root cause to why your battery is not charging, you can have the alternator tested at any local auto repair store free of charge.

Fuses
Check all of your fuses first, it's always best to start out with a cheaper solution and then work your troubleshooting up to the more expensive items.Sometimes, one fuses can control multiple circuits, so always make sure to check them all, under the hood and inside under the dash. If all else fails, stop by your local auto service shop and I'm sure they will be able to take care of the problem in a timely matter.

Ground Wires
Check all of your ground wires on the starter,chassis,battery terminals & engine/transmission. A short in the ground can prevent the vehicle from starting and cause the engine to skip/miss fire. Even had a quick check on the grounding electrical panel where the ECU bolts up to, could have a bad connection.

Starter Remote Wire
A starter remote wire is a wire that a signal travels from the ignition control switch "aka" the car key to the starter, if that wire is loose,corroded or unplugged/unbolted the vehicle will do nothing when the key is turned.

Starter Solenoid
A bad starter solenoid can be another reason your starter does nothing when you turn the key, the solenoid can be removed from the starter and replace for a fraction of the cost of the entire starter. Your local auto part store can test this for you if you bring them the entire starter. In order to just replace the solenoid you'll more then likely have to order it online, local stores only carry the entire start and solenoid as one unit.Sometimes tapping the solenoid with the back of a screw driver or rubber mallet while someone turns the key will make it start, that will tell you it's the solenoid for sure.

Stuck Bendix
You're probably asking yourself "a stuck what??.. A Bendix is the drive gear the starter shoots out to meet with the flywheel so it can turn the engine over, once the engine fires the Bendix contracts back into the starter. In some cases the Bendix can be lodged on the flywheel.Some causes could be rust or the last compression cycle held the flywheel in place when the Bendix shot out, in any case it could be a possibility. Pull the starter and see if the Bendix is sticking out,that should tell you.
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"Vehicle Cranks But Won't Fire Up"

Main Relay
The main relay is the relay that triggers the fuel pump to power up, usually when you turn the key you can hear the fuel pump prime if you listen carefully. If you don't hear it then it could very well be the issue. Common signs are, when it's hot outside it tends to not want to start more then when it's cold or your just running the vehicle for the first time.

Fuel Cut Off Switch
Most vehicle don't have one but the majority of for truck do, the switch is usually located by the passenger sides right foot on the floor board under the carpet or under the  passenger kick panel. If accidentally pushed in it act as a emergency shot off and will not all the fuel pump to power up by killing the power to the main relay, you will crank all day and not even a miss fire will happen.Simply push the button"it should be red" every vehicle may vary, once push in that should rest the system and allow fuel.You want the button "DOWN NOT UP".

Fuse
Every vehicle has a fuel pump fuse, check your manual to find where it's located (usually under the hood) and look for a split or brunt signs, if your unsure just replace it with a new fuse of the same amp rating.

Bad Cap & Rotor
A bad distributor cap and rotor will prevent the vehicle from firing up. it's best to replace them when you change your spark plugs and wires. You can always try to clean the terminals inside the cap with a brass wire brush or fine grit sand paper and then the top of the rotor and the tip that sends the voltage to the cap to see if it starts to improve starting.

Distributor Coil
If the distributor coil goes bad your vehicle will never start period. You can preform test with a multimeter testing it's positive receiving lead & sending lead to make sure it's getting 12volts from the ignition & sending it to the rotor. if it is then you can always pull a spark plug out attached to a plug wire and have a friend turn the engine over while you carefully touch the spark plug  to a valve cover nut/bolt and see if your getting spark They also sell a spark plug tester tool if your a chicken butt :) I hate getting jolted myself too.

Ignition Switch Control Module
I this goes out you wont send any power to the starter or the coil pack in side the distributor or coil packs. Replacing it is your only option.

Clutch Sensor/Switch
This is a sensor/switch located above your left foot above the clutch pedal. When you press the clutch in it tells the ECU "computer" to allow the vehicle to start, a lot of times a little rubber grommet pop's out of the clutch pedal and the sensor button never get compressed because there's a hole where the rubber grommet was. replacing the grommet or switch could fix your starting issue.To test this, put your vehicle in neutral and push the button in on the sensor, then turn the ignition key.

Automatic Transmission Range Sensor
This sensor lets your Ecu "Computer" know when it's in park so it can allow the ignition switch control module to send voltage to the starter. If this device is shot, the vehicle will not start or only start sometimes when you slam the automatic shifter in park.