Dumb question: did you check the fuses?
If so, check the CONNECTIONS in and out of the fuse block, a they often are mungy.
If that looks good, check your switches - they actuate using teenie-weenie bitz-o-brass and it doesn't take much to corrode tem and break connectivity (WARNING - dissassemble switches CAREFULLY with a piece of paper at-hand to note how things fit together as well as on a big sheet of newspaper, as some parts are held in by springs and WILL go shooting off into oblivion otherwise when un-screwed ... don't ask me how I know this). ???
If that all looks solid, you need to start at one end of the circuit (I like starting at the bulb end personally), then trace the circuit alllll the way back to the battery, looking for rusty spots (which could be where a steel part corroded and pierced/frayed the wire jacket), kinks/crimps (the wire may be severed inside the wire coating and you can't see it), etc. Broken wires can almost always have the damaged part cut away and repaired using crimp-on "butt caps" (the 'Shack has 'em) - do yourself a favor and DO NOT just twist the ends back togehter and wrap w/ electrical tape or pretty much guarantee that some day when you're 200 miles from home (in a rainstorm typically), the tape will pull away, the connection will ground, and you'll pop a fuse (which you won't have a spare for), ruining your otherwise perfect road trip, all because you were too cheap to buy a $1.29 pack of butt-caps.
If the wires all look smooth/solid, check all places where a wire is crimped into a bullet or other connector by tugging on it firmly - if the connector wiggles or, worse, slides of, replace it with a freshly-crimped connector (cost less than a buck usually at Radio Shack).
When re-assembling connectors, do yourself a favor and use a tiny smear of dielectric grease on the male connector (you can get a tiny foil pouch plenty-big enough to do every connector on the bike 6 times over at any Autozone in a bin beside the register for usually $.50) - this will not only seal the connection against moisture, but make later disassembly a snap as the connection will now be lubed.
Good luck!
