Hi Scott
I am assuming the battery was ok too? When the RAC changed the spark plug did they test for the spark? (also the gap at the spend of the spark plug has to be set correctly for your bike).
OK here's what you can do. First you need to make sure there's a spark. When you turn the ignition the starter motor should turn over. If it doesn't then the battery may need a recharge. If it does turn over then next check for a spark. Remove the spark plug from the engine and put it into the cap that connects to it when it's in the engine. Hold the plug so that the gap (spark) end is touching the metal frame of the bike and then turn the ignition over. You should see the plug spark with a healthy spark. If it doesn't spark then the problem is electrical and could be a fault with the solenoid (easily cheaply replaced) or some other part of the ignition circuit. And that is something that could have gone whilst you were riding to work and so would make sense. To find a fault like that for yourself..buy a Haynes Manual for the bike and work through the chapter on Ignition System fault finding.
If there is a spark then the next thing to check is that fuel is getting to the engine. Make sure the pipe from the fuel tank to the carburettor is not blocked. Disconnect the fuel pipe from the carburettor and make sure petrol is running freely from the tank. Use the tap to control the flow. Any sign of restricted flow can lead to fuel starvation and hence poor ignition. If that seems fine then check the air filter isn't clogged. It's unlikely the carb will be clogged as it was running 8 hours before. The only other thing that can disrupt fuel flow are sticking valves. It's worth checking there's no split in the pipe that connects the carb to the engine as well. The fuel is driven by a vacuum so any kind of air leak between the carb and cylinder head will affect the ability for the engine to start.
For future reference, a bike can be bump started in the same way a car is. Take it to a hill. Put the bike in second gear and keep the clutch in. As you rool downhill on it and pick up some speed release the clutch and the engine should start if there's nothing wrong there. Useful to know when the ignition fails in the middle of nowhere! (given that most bikes no longer have kick starts)
If none of the above solves the problem then come back. There are other things to check beyond that which get into the realm of engine/ carb fault finding but hopefully the fault is a simple one to find as above.
Angela.
February 2008