Got power to the battery, starter, ignition, everywhere, but no lights, horn electric start... nothing. Everything is dead. '75 Superglide FXE Harley.

When I used to work on the coast at Point Magu, CA, my motorcycle would sometimes do that. The problem used to be a thin, invisible film of corrosion on the battery terminals. Try cleaning the battery terminals and connections and see if that solves the problem. Also, you might check all fuses with a test light or multi-meter. I would also remove and fully charge the battery, at a rate not exceeding 2 amps. Finally, make sure the "kill" switch is not turned on. I've even seen a bad kill switch prevent a cycle from starting.

What Vehicle? You have power to Ignition, Do you have Power out(Ignition,Start)?

Where exactly did you measure everything... and what did you use to measure it. What voltages do you have? Since the battery cable goes to the starter relay (and there is a second thick cable to the starter solenoid) before branching out to the circuit breakers, unless its been rewired differently, this has circuit breakers and not fuses which can and do go bad sometimes... what does it measure there with the switch off, and again with it on?

Sorry I missed the smaller print saying it was a Harley. Doesn't Matter as Many Starting systems are the similar
A Good Ground from Battery to Engine Block,(Starter). Then a Large Red(Positive Cable to Starter), Then you will have a small wire(positive) from Ignition Switch to Energize the Solenoid. If all conditions are met and when you turn Ignition and power makes it to the small Start wire at Starter, and it doesn't start, then Starter is Bad. You do need a good Battery, Good Hots and Grounds. Some Harleys have a TSSM(Turn Signal Security Module), which disables starter and Ignition. Some Starters need the Clutch Pulled In, or may have other safetys disabling the Starter. Good Luck.