1999 E-TON TXL-50 (won't start)
 

My son was riding his ATV one day, the next it wouldn't start. Turns over fine. I have checked oil and fuel levels. They are fine. Checked the ignition to make sure key is on and have ensured that the tethered plug in the back is inserted properly. I'm not sure what direction to take now. I'm thinking it's electrical, but not sure how to find out. Any help would be greatly appreciated. Thanks.

here some starters
 

pull plug boot and turn over a few times. if there is fuel on plug problem is electrical if not it is fuel. check fuel level and filter

Never pull the wire off the spark plug and crank the engine with spark enabled. You can wreck the coil and/or the CDI. Without a narrow spark gap for energy in the coil to jump and safely get dissipated, the voltage on the coil secondary will rise to tremendous levels. Something has to give, and what often happens is the coil insulation breaks down and the spark jumps to ground inside the coil. This forms a carbon path which shunts off spark energy even when the plug is reconnected. You end up with a weak spark (or no spark). If the coil breaks down inside from the secondary widing to the primary winding, the path to ground is back through the CDI - taking it out instantly as well.

If you do pull the plug wire off, you may get away with it. But you may not. It is not worth the risk.

A much better way is to leave the plug hooked up and turn on a kill switch (disabling spark). Crank away, then pull the plug and look for it being wet with fuel.

Do you have spark? Procedure:

Pull the plug, reconnect it to the plug wire. Hold the plug threads against engine ground while cranking the engine with all kill switches in the "run" position.

You should see about 10 bright and steady sparks per second while the engine is cranking.

If you don't have spark find a voltmeter and we look further. If you do have spark then you have fuel or compression issues.