First of all, try using the choke to kill the engine if the switches won't work. If that doesn't work (you should check your choke), try shorting the spark by resting a screwdriver across the top of one spark plug boot and touching the top of the engine. Make sure its a well insulated screwdriver or you'll get "bit" with quite a shock. If the screwdriver is well insulated, though, you should have no problem.
As for the problem, the common ground wire has probably either separated from the plug or broken. One way or another, you're not getting a ground. Check all of the black wires going into and out of the main electrical plugs. If nothing is evident, check the plugs themselves. I had this exact problem a few weeks ago. One of the black ground wires was not making contact inside the plug that connected the main wire bus from under the airbox to the engine. I simply pressed the wire further into the plug until it made good contact and then used a little epoxy to hold it securely in place in the plug. At some point in the off-season, I'll replace the plug.
