A Black/White wire is the classic color for a kill switch wire. It should not be grounded. No need to look at the CDI yet, a ground on the kill switch pin of the CDI will prevent spark. That has to be fixed first.
Disconnect the black white wire at the CDI. Does the quad start now? Important: Make sure you can hook that wire back up quickly to kill the engine in an emergency, and note there is normally high voltage (more than 100 volts) on this line, so you can't do this with your fingers. Have some insulated pliers or a clip lead standing by.
My 150cc quad has 3 kill switches:
Ignition switch: One pole of two shorts the kill switch wiring to ground when the ignition switch is turned off. This is what stops the engine when you turn off the quad with the ignition switch.
Tether Kill switch: A cord pulls off a plug which shorts the kill wiring to ground. The plug holds a switch open (no connection to ground) when installed, allowing spark. [To disable this switch simply disconnect it. Don't short the wires together - that kills the spark - not what you want to do.]
Left Handlebar Kill switch: When clicked this shorts the kill wiring to ground. On many quads there is a two step process to reset this switch. You must place the switch in the middle position, and then push in (towards the handlebar) until it goes *click*. Anyone who has ever had this style switch has been had at least once by neglecting the "click in" part.
Here is a wiring diagram of my 150cc quad. There wasn't any available diagram when I bought it so I traced out my own:
So why was your battery dead? Is your charging system working?