#5: It Just Went Dead
If you're lucky, someone simply bumped the kill switch. Or you could be out of fuel. If neither of these checks out, this usually represents some type of electrical failure. It could be a blown fuse or tripped breaker, a loose connection or corrosion.
Solution: Start with the simple scenarios. On any boat equipped with a kill-switch and lanyard, make sure the lanyard key hasn't come loose. Sometimes, it might seem to be engaged, but has actually slipped just enough to activate the switch.
Ignition switches can also fail or suffer loose connections, and though this will mostly likely show up at start-up, it's worth fiddling with the switch a bit (and checking its attendant breaker or fuse) before moving on to the engine side of things.
Back at the business end, where the big wires live, corrosion is your most likely source of problems. Even boaters who contentiously maintain the battery terminals might forget that there's another end to those wires, and they also require the occasional cleaning.
If it turns out to be something more complex - such as an ignition chip on an EFI engine - you might have to pull out the cell phone or put out a call on channel 16. Prevention: Learn the various components of the ignition system, and periodically inspect, clean and coat each exposed connection with an anti-corrosion product.
Carry Onboard: Wire brush to clean terminals and Corrosion X spray.