gG'day Rangie-4x4,
Welcome to the forum.
Please provide a little more information for our members.
Is it a manual or auto etc.?
Are the plugs dry?
If so can you hear the pump when you first turn the key?
Is the spark bright?
In the dark can you see any arcing when it's cranked over ?
Coolant sensors are a problem. Won't start if open circuit, you can substitute a 200 ohm resistor to try, before you replace.
Auto's have a switch on the gearbox to prevent starting when in gear these can cause your problem.
From this little list you can see why we need as much iformation as possible.
Good luck with finding your fault.
Fred