Most likely a dead coil. Happens all the time. What kind of tractor is it?...doesn't matter, look up the model number of the engine (it's on the engine)...
then:
http://www.briggsandstratton.com/maint_repair/
manual_and_more/
pull up a p/n and go on ebay or whatever, probably cost ya about $30 or so on ebay.
I think I've replaced... four(?) on assorted B&S engines (or as I call them BS engines

)