When your motorcycle won't start – maybe won't even crank over – it's best to keep your cool and work through the problem logically, rather than, say throw it in the pickup truck too visit the nearest motorcycle garage.
The most common kind of 'no start' occurs when the motorcycle won't even crank over. You turn the ignition key, punch the starter button and nothing happens. Chances are it's something very minor, so let's work through it.
Start by checking the most obvious things. Do you have juice? Do you notice the normal indicator lights glowing? His is your first indication you have a live battery. Watch these lights (or headlights) and see if they dim when you hit the starter button. Understand this distinction: If the lights dim, you have a weak battery. If the lights (particularly the headlight) go all the way out, you have confirmed that the starting-safety circuit is good up to the starter solenoid. It's normal for the headlights to be turned off by the starter-switching circuits to save juice for the actual start-up (aren't motorcycle designers smart?).
Let's say the battery seems to be fully charged. Is the engine-stop (or kill-) switch in the right position? Is the motorcycle in neutral gear? Are you pulling the clutch in all the way? Many newer motorcycles have a clutch interlock switch to prevent the starter from engaging unless you pull the clutch level, while others have a clutch switch that interrupts starter operation only when the sidestand is down or the motorcycle is in gear. On some motorcycles, you simply cannot start the engine if the motorcycle is in gear or the sidestand is down, regardless of clutch-lever position. Before you start tracing circuits, cycle the ignition and engine-stop switches, put the motorcycle on the centerstand, or merely straddle it with the sidestand up, and pull release the clutch lever a couple of times. By doing this you may succeed in cleaning slightly corroded contacts, which would cure the starting problems.
Still no luck starting the motorcycle? Start looking a bit deeper, Glance at the connection to the handlebar switches. Make sure the connectors are solidly on the pins of the clutch-interlock switch and make sure the sidestand switch is not damaged or disconnected. Locate the starter solenoid and place the (red) positive lead of your voltmeter on the battery side of the relay. The black (negative) wire from the volt meter can be grounded to the frame. You should see battery voltage there. Now check the other side of the relay, there should be no voltage there until you push the starter button. If there's no voltage when you push the button, the problem is upstream; if there is – and you'll probably hear the relay click – you've got either a duff starter or a bad connection. Pay particular attention to the engine ground leads, the connections from the chassis to the engine proper.
But what it the battery seems dead? Before reaching for the charger, check a couple of things. Gain access to the battery and make sure the connections are tight and free of corrosion. Next, put your voltmeter on the battery terminals; red to positive and black to negative, but you knew that, right? The voltmeter should read 12.6 volts, minimum, but 13.2 volts is the right voltage for a fully charged battery. Now punch the starter button. Does the voltage drop dramatically? If it does, the battery is weak or you have a bad connection from the battery to the starter. At this point, your best bet is to put the battery on a charger. You could push the motorcycle to the top of a hill, or ask a few buddies, and bump-start the motorcycle, but the rapid charging of the battery once you get the motorcycle running will definitely reduce its life; trickle charging is much kinder.