See if it starts by using the recoil starter. If it does,could be as simple as a dirty or out of aligned pick up coil. Similar to what happened to this member. no spark to plug when using electric starter - Page 3 - ATVConnection.com ATV Enthusiast Community
If it doesn't crank,then pull the plug and ground it to the engine and crank it over and check for spark with a new plug. If no spark then you'd need to trace things a little further. One thing you could do is to pull the black wire from the cdi which by passes the kill circuit and see if it has spark then.