Seized engine? Weak or dead battery? Loose or corroded battery cables? Flat spot on starter motor? Bad starter solenoid? Bad neutral or clutch safety switch? Bad ignition switch?
It will probably be your starter motor Starter is out!
One of a few things depending on the click. If it makes consecutive clicks when you turn the key (click,click,click,click) then the odds are the battery is dead or in need of charging and there just isn't enough charge to fully engage the starter and turn over the engine. If it clicks just once, the selenoid is usually bad, or the starter is "locked" and will need to be replaced.
I'm guessing it clicks but won't start? If that's the case its the starter drive going out to engage the flywheel but the starter motor is not turning. Probably needs a starter but you should check to make sure there is correct voltage in system (battery & alternator is o/k) and that the wiring is also o/k to the starter & at the battery.
Look for a loose or corroded battery cable. The problem may also be a faulty starter solenoid/relay switch - you might want to have it checked out.
Electric current from the battery goes to solenoid, it clicks in and connects the full power of battery to starter main terminal, this spins the starter motor over, the starter Bendix gear on the end of the motor shaft spins around and forward about an inch to engage the gearing on the rim of the flywheel. This turns and when speed is up it throws the starter gear back to it's rest position.
To give a generic answer, it kinda depends on what type of click. If it is a rapid click-click-click-click... it is probably a dead (or near dead) battery. If it is just one solid "click", it is probably your starter. The single click is the sound of the selinoid clicking in to try to engage the starter. If the starter is faulty, it won't engage and turn your engine over to start.
bad solenoid in the starter or insufficient voltage to the starter
Does it click, sometimes or all the time. Clean all the power leads to make sure for good contact. If it still clicks and starter does not engage, then change it. Not an expensive part.
Replace the starter
sounds like the starter solenoid, you will have to replace the starter
The starter is located just behind the top end of the engine. I have a '99 king quad. Mine solenoid clicks too. I thought it was a loose wire because sometimes it would turn over but I took it in and was told i need a new stARter. they run about $75.
if the battery and battery conections are good, then you need to replace the starter motor