OK so you can confirm that you are getting good spark to all cylinders. Thats good, you are about to move on to checking for something else. However remember two things:
First, the spark plugs are also a part of your sparking system. If your mixture is off and you are running rich, they can become so fouled that they will not fire, even if they are getting good spark. Pull a spark plug and inspect it. Here are four typical spark plugs which represent different running conditions. Click on the pictures to see a large image:
Oil fouled. You are burning oil and the residue is coating the spark plug. This may also happen in 2-strokes which are running rich or improper mixture. This spark plug will have a hard time firing.
Carbon fouled. An overly rich mixture is coating the spark plug in black carbon. This can happen in any bike which has a fuel:air ratio that is off, due to dirty air filter, improper jetting, improper float height, poor gasoline quality, stuck floats, etc. This spark plug will have a hard time firing.
If your spark plug is one of the above, the best repair would be to address the rich or oil condition, and then replace with a new plug. However you may be able to get some extra life out of the plug by cleaning it up with a small file or piece of sandpaper.
Good spark plug. This is what it should look like, brown in color and clean. You won't get any residue on your finger when you rub the tip of the spark plug.
Lean. This plug probably won't have any problems firing, but the ashy white deposits indicate you are running too lean. Lean means hot, and will put undue stress on your engine. Correct as soon as possible. For help diagnosing the causes of a lean mixture, head on over to the "air" portion of this diagnostic algorithm.
OK so if all the spark plugs are good and not fouled with either oil or carbon, assess your spark plug boots. If they are wet or damaged, spark may escape and ground out to the frame. If you can get your bike to a very dark place, or if it is nighttime, take off the tank and turn over your starter motor. Look all around the boots for tiny sparks which may be escaping to the frame, if they are there you should see them in the dark. Time to replace the boots or the spark plug wires altogether.
It is not very likely that your bike will jump time, at least less likely than in a car. I don't generally consider it for emergency diagnostics, unless it is on a points bike. If your bike has points, access the points plate and see if it is loose. If it is tight then you are probably still ok on your timing. If it is loose, you will need a timing light to reset it properly, or follow the timing procedure according to your make/model. If you are in a jam you can always just wiggle it back and forth and experiment until you get it close enough to run the bike.
If it all checks out you can move on to a different branch of the algorithm.
This is a typical points plate on a motorcycle, found under a small panel, generally on the upper left side of the engine: