The truck will turn over, but won't start - 2004 Mazda 6

Check the throttle. Make sure it isn't sticking shut preventing air from getting in the cylinders.
Are you getting spark? If the spark plugs aren't new, pull one out and look to see if it is bad. If it looks pretty crummy, replace them and try again. Check to make sure the distributor and wiring harness to the spark plugs aren't falling apart or disconnected.
Check the compression in your cylinders.
Check the gas flow
You need spark, gas, and compression (involving air) to get a turning over vehicle to fire up. Check all connected with these, testing with each change, to try to isolate the problem. Those listed above would be my starting point. It looks like a relatively new vehicle, so i would start with checking the spark plugs to see if there is anything awry with your cylinders (this spark plug diagnostic chart could help http://www.ingfatrygg.se/Spark_plug_diagnostics.html )