"The Equalizer", a novella by Jack Williamson; first published in Astounding Science Fiction, March 1947, available at the Internet Archive. Any of these covers ring a bell?
A ship comes back from a long mission.
Interstellar Task Force One was earthward bound, from twenty years in space. Operation Tyler was complete. We had circled Barstow's Dark Star, nearly a light-year from the Sun. The six enormous cruisers were burdened, now, with a precious and deadly cargo—on the frigid planets of the Dark Star we had toiled eight years, mining raw uranium, building atomic plants, filling the cadmium safety-drums with terrible plutonium.
Their base/home planet is largely deserted and they don't know what happened.
"Something has happened to the Directorate!"
"Eh?" Cameron's veiled amusement vanished. "What?"
"Here are the facts." Heavily, Hudd lurched forward against the desk again; his voice had a brittle snap. "We began calling Fort America weeks ago, from millions of miles at space. Our signals weren't answered. So far as we can determine, the moon has been abandoned."
His bloodshot eyes looked haunted.
"We haven't tried to signal the earth—I want to keep the advantage of surprise, until we know the situation. But things have happened, even there."
He reached, with a huge and hairy paw, for the little golden bust of Tyler and resumed his nervous drumming.
"But we've been listening, on every possible wave band. Of course, out here, we couldn't expect to get much. But we are in range of the great television propaganda stations of the Applied Semantics Authority—and they are dead. All we have picked up are feeble clicks and squeals—scrambled radiophone signals, apparently, which our engineers can't unscramble."
They search for people and find a farmer.
A fern-grown ravine let us down from the low cliff. We waded the clear stream and climbed again through the woods beyond. Nearer the dwelling, the land had been cleared. We crossed an orchard of young apple trees, slinking toward the voices of the man and the woman.
The farmer proceeds to demonstrate that she has unlimited energy at her command by shooting at their ship in orbit, possibly shooting it down, with a hand-held device.
No, one of the spacers shoots first, killing the farmer and his wife:
"Stop 'em!" whispered Lord.
Instantly, the automatics crashed. The girl crumpled down beside the bright machine. The man ran another step, uttered a strangled cry, fell sprawling on top of her.
Later, the slain couple's friends and neighbors come to arrest the murderer; when he tries to get away in a spacecraft, they shoot it down:
The tall man turned, with his gaunt face grimly angular, and watched the life-craft go. It became a vanishing point of bright metal and violet fire. Its thunder rolled away.
His clumsy-seeming weapon lifted at last, and clicked.
"Down, Barstow!" the girl screamed at me. "Cover your eyes."
Astonished to find that I was left standing alone, I dropped. The flash of heat stung my skin. I looked, afterward, in time to see the small bright cloud of iridescent metal vapor fading in the blue northward sky, turning into a white tuft of rising cumulus. The crash came a whole minute later, like one loud peal of thunder.
They talk to her and find out that someone on the planet discovered that if you take a copper wire and twist it in certain ways, it produces unlimited energy.
"Barstow's dream—like my own—was merely a simple way of doing a simple thing. Material energy exists, as Einstein demonstrated. Barstow dreamed of a simple way to let it flow. The equalizer is his dream, realized."
I couldn't help the breathless interruption:
"Just a solenoid." Cameron nodded. "But wound in a certain way, not helically, so that its field slightly alters the co-ordinates of space and slightly changes the interaction of mass and energy. The atomic particles of the solenoid are equalized, as your father termed the process. The converted energy appears as direct durrent in the wire.
"The fact is simple—even though the tensors of a new geometry are required to describe the solenoid field. That apparent complexity is more in our awkward description, however, than in the vital fact. The actual specifications of the equalizer can be memorized in five minutes."
This plus their existing technology meant that civilization collapsed, because everyone could make or destroy anything easily.
That's one way of looking at it. Another is that the dictatorship has been overthrown and replaced with a decentralized anarchist utopia.