My memory is a little fuzzy on this one, so I may not have all the details right. As best as I can remember, the issue went like this:
A humanoid alien (possibly a superhero of some sort in his own right) lands on the Earth and is encountered by the X-Men. There may have been an initial conflict/misunderstanding, but it soon becomes clear what the alien visitor needs. His planet has a ring-like sun surrounding it, one that has failed. There is a device that can be used to restart/recharge it, which was placed there, or at least activated by, some of Earth's superheroes in the past. The first time the ring-sun was recharged, Thor used Mjölnir to generate the power needed to activate the device. None of the X-Men have near the power Thor has, but time is short for the world, and they agree to help as best they can.
The alien world, meanwhile, has devolved into chaos, or perhaps a civil war. The planet remains habitable despite the failing sun-ring, but darkness has descended on the world. The X-Men are forced to battle their way to the device. This proves a challenge, especially for Cyclops. With no sun to provide solar energy, he has no way to recharge his eyebeams. He gets progressively weaker through extended combat, his final eyeblast being rather paltry. I believe Wolverine asks "are you okay, Psyke? That last beam looked kinda...". To which Cyclops tells him to shut up, or something.
Eventually they reach the device, and try to figure out how they can activate it. Cyclops has no power left, but it is presumed he would have the best chance to activate the device if he did. His solution is to have Storm hit him with lightning. Colossus would ground her, allowing her to amplify her power from the planet (or something like that). Cyclops, for his part, would try to absorb the energy from Storm and somehow metabolize enough of it to power his eyebeams before he is electrocuted.
The resolution to the crisis comes when Cyclops is able to absorb enough of the energy Storm is hitting him with the let out a single, super-powerful blast. The beam is so powerful, it lacks the usual crimson color. Instead, it is a blindingly bright white beam, one more powerful than Cyclops has ever fired. It is enough to recharge the device, which in turn is able to restore the power of the ring-like sun surrounding the planet.
Late 70s to maybe very early 80s? I think it came out a little bit before the Hellfire Club/Dark Phoenix sagas. I seem to remember it being a longer than normal issue, meaning it might have been some sort of "special", rather than a regular issue of X-Men. I also distinctly remember that Phoenix/Jean was nowhere in the story.