OK so you were just riding along, the bike broke down, and now you cannot start it. You cannot get a spark out of the screwdriver test on any cylinder. Your battery has good juice, and the starter motor turns over just fine, but still no spark.
First, check all your fuses. If you find a bad one, replace it and see if that solves the problem.
There are two main types of motorcycle fuses, and they are generally found in a plastic fusebox under the seat or behind a side panel:
Now, check if you have a kickstand switch or a clutch switch. Here is how to check if they could be causing the problem:
Now, take off your gas tank and seat, and inspect your wiring harness. It is possible that a connection has come undone. Pay special attention to the wires that come out of the left side of your engine, they will often run up towards your main wiring harness, or CDI box. If you have a points bike (often those made before 1980), inspect those wires.
Check your kill switch (make sure it is in RUN position). If you have any doubts that it is functioning properly, locate the two wires that run to it, and cut them. Try different combinations of twisting them together, or separating them from each other (different kill switch models work in different ways).
Your coil (or coils) are usually located under the tank, shaped like a fat cylinder. Often they will have 2 small wires running into them, and one or two fat spark plug wires running out. Make sure the small wiress to the coils are attached, and the fatter spark plug wires are firmly attached to both the coil and the boot at the other end. Inspect your coil for cracks. Sometimes small cracks can heat up and turn to bigger cracks, and then your spark can escape. Speaking of escaping spark, check the integrity of the spark plug wire boot and the line to make sure your spark is not grounding out to the frame.
Your ignition cylinder may also be to blame. A simple test is to try hotwiring your bike and see if that solves the problem. Here is how to hotwire your bike:
Your coil or CDI might be failing. Here is how to check for that:
If you try all this and still no spark, it may be time to truck the bike home and get deeper into your electrical diagnosis. Here is a video which shows how you can use your wiring diagram to diagnose a fault: