If you turn the key on and absolutely nothing happens, no lights, horn, or anything at all.
1. Dead battery, but they don't usually just quit out......However on that note..... many a new Ducatista have been known to "accidentally" turn their key all the way to the left to lock the bars before removing the key. This position is a park condition and can leave the back light on quickly draining the battery.
2. You blew a fuse. Check the main fuse, it certainly won't be an accessory item that blew.
3. A battery cable came off. It happens. Maybe the previous owner did not tighten the bolts enough. Check the ground wire where it attaches to the frame that it did not break or come loose somewhere you wouldn't think of.
4. The ignition wiring harness has come loose or disconnected due to some fiddling. Trace them back.
5. Voltage regulator is toast. I've heard that when they go, they can burn up a diode that separates the battery from the rest of the bike. The diode is a one way check valve and if it gets burned up can allow the battery to discharge when not in use.
I can't think of anything else.
Do you have a battery charger? If so, hook it up for an hour or so and check to see if anything works when you turn the key on. If it lights up, remove the charger and see if things still work.
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