CM400 Ignition stopped working after spark plug wire disconnected while running

When you press the start button it should disconnect power from the headlight and supply power to the starter relay. Since your lights work you must be getting power to the start button. Does the headlight go out when you press the button?

Your ignition system (CDI) is separate from the rest of the electrical system, it has nothing to do with the starter or the lights/horn.

Welcome to the group.

Got pictures?

Then you need to check whether you are getting power to the starter relay. The yellow/red wire at the relay should be 12v from the start button. The green wire is connected to ground. If you are not getting power you may have a problem with the start button or with the wires from there to the relay. If you are getting power there make sure you have a good connection to ground from the relay. If both those are good the relay should "click" when you push the button. If it does not "click" the relay may be bad. If it does "click" you should have battery voltage at the large relay terminal that connects to the starter and at the starter.