I have no spark. The thing was running one day and not the next.

 

Are you sure you haven't knocked the kill switch to kill? Surprising how many bikes I get in to repair where the owner has knocked it by mistake, and must admit I have done it myself, only finding out when checking the wires to the CDI. The cause can be anything in the ignition system, try a new plug, if that doesn't work get the workshop manual, this tells you how to check those wires to the CDI and pin point which one has a fault or, if they all test OK, you change the CDI.