I was riding the other day and the quad just died. Would not restart so I bought a new plug and noticed I had zero to little spark. When I checked the values of the coil with a multil tester it shows that its bad. Do coils normally go out with out warning.
Sometimes it's just the sparkplug boot thats bad. You might be able to pull the boot off and check the coil again, if you get good #'s get a ngk boot to replace the old one.