I would also check the ground wire. Make sure it is solid-no breaks and has a corrossion free mounting area. Then start checking contacts where the connectors join together. ATVS get a lot of jarring action and this could be a problem.
The issue could also be with the ignition switch itself.
I doubt it is the coil, as they usually don't "just drop dead" like you state that "it ran fine and just quit".
Get a service manual for the ATV and you will have all the electrical specs you need to do a good job. If you can't find the problem after running down a trouble shooting list in the manual then holler back.