I believe you're thinking of "Forgetfulness", a novelette by John W. Campbell, Jr.. (This story was also suggested as a possible answer (not accepted) to the question Short story where advanced civilization doesn't understand their technology.) It was originally published under the pseudonym Don A. Stuart in Astounding Stories, June 1937, which is available at the Internet Archive. It has been reprinted many times; any of these covers look familiar?
The story is much as you described it, except that the space explorers are humanoid aliens; the planet is Earth (spelled "Rhth" in the story), and the natives are humans of the far future. Also, the spaceship does not need repairs.
In the story, humans encounter a race of aliens on a planet that seem very simple. They live in plain huts and don't use any technology.
It was triumph, for six long years of travel, at a speed close to that of light, lay behind them; three and a half light-years distant was Pareeth, and the crowding people who had built and launched the mighty two-thousand-five-hundred-foot interstellar cruiser that had brought this little band of one hundred. Launched in hope and striving, seeking a new sun with new planets, new worlds to colonize. More than that, even, for this new-found planet was a stepping-stone to other infinities beyond. Ten years of unbroken travel was the maximum any ship they could build would endure. They had found a planet; in fact, nine planets. Now, the range they might explore for new worlds was extended by four light-years.
And there was sorrow there, too, for there was a race here now. Ron Thule turned his eyes toward the little clustering village nestled in the swale of the hills, a village of simple, rounded domes of some opalescent, glassy material. A score of them straggled irregularly among the mighty, deep-green trees that shaded them from the morning sun, twenty-foot domes of pearl and rose and blue. The deep green of the trees and the soft green of the mosslike grass that covered all the low, rounded hills made it very beautiful; the sparkling colors of the little gardens about the domes gave it further enchantment. It was a lovely spot, a spot where space-wearied, interstellar wanderers might rest in delight.
They find the ruins of great abandoned cities that the ancestors of the aliens built and think how sad it is they have reverted from their past golden age.
The city flamed before him. Across ten—or was it twenty—thousand millenniums, the thought of the builders reached to this man of another race. A builder who thought and dreamed of a mighty future, marching on, on forever in the aisles of time. He must have looked from some high, wind-swept balcony of the city to a star-sprinkled sky—and seen the argosies of space: mighty treasure ships that swept back to this remembered home, coming in from the legion worlds of space, from far stars and unknown, clustered suns; Titan ships, burdened with strange cargoes of unguessed things.
And the city peopled itself before him; the skies stirred in a moment's flash. It was the day of Rhth's glory then! Mile-long ships hovered in the blue, settling, slow, slow, home from worlds they'd circled. Familiar sights, familiar sounds, greeting their men again. Flashing darts of silver that twisted through mazes of the upper air, the soft, vast music of the mighty city. The builder lived, and looked out across his dream—
But perhaps, from his height in the looming towers he could see across the swelling ground to the low, rounded domes of his people, his far descendants seeking the friendly shelter of the shading trees—
[. . . .]
Ron Thule looked down at them, and a feeling of satisfaction and fulfillment came to him. Pareeth would send her children. A colony here, on this ancient world would bring a new, stronger blood to wash up in a great tide, to carry the ideals this race had forgotten to new heights, new achievements. Over the low hills, visible from this elevation, lay the simple, rounded domes of the people of Rhth—Seun and his little clan of half a hundred—the dwindling representatives of a once-great race.
It would mean death to these people—these last descendants. A new world, busy with a great work of reconquering this system, then all space! They would have no time to protect and care for these forgetful ones; these people of Rhth inevitably would dwindle swiftly in a strange, busy world. They who had forgotten progress five millions of years before; they who had been untrue to the dream of the city builders.
The twist is that the aliens have developed mind powers far beyond the relatively crude technology of the past
"Once"—Ron Thule's voice was tense—"the city builder made atomic generators to release the energy bound in that violent twist of space called an atom. He made the sorgan to distribute its power to his clumsy shells of metal and crystal—the caves that protected him from the wild things of space.
"Seun had forgotten the atom; he thinks in terms of space. The powers of space are at his direct command. He created the crystal that brought us here from the energy of space, because it made easy a task his mind alone could have done. It was no more needful than is an adding machine. His people have no ships; they are anywhere in space they will without such things. Seun is not a decadent son of the city builders. His people never forgot the dream that built the city. But it was a dream of childhood, and his people were children then. Like a child with his broomstick horse, the mind alone was not enough for thought; the city builders, just as ourselves, needed something of a solid metal and crystal, to make their dreams tangible."
and are able to send the humans back in time/space with just their brains.
"My son was born in space, and is four. Yet we were gone but a single year from Pareeth." Shor Nun sighed.
"Our fleet took six years to cross the gulf of five light-years. In thirty seconds, infinitely faster than light, Seun returned us, that there might be the minimum change in our racial history. Time is a function of the velocity of light, and five light-years of distance is precisely equal to five years of time multiplied by the square root of minus one. When we traversed five light-years of space in no appreciable time, we dropped back, also through five years of time.
