In my story, a group of human colonists are set to colonize a distant planet. But on their way there, the fusion drive on the ship stop functioning when they enter an uncharted system, and they crash on a planet. Some of the colonists manage to survive and eventually figure out that there's a mysterious "field" that's preventing fusion from happening. The field was deployed by an ancient civilization on the planet, and it is expanding and will eventually extinguish the sun.
So my question is, what are the plausible mechanisms behind such a fusion prevention field? My story is not "hard" sci-fi and we can assume that the said civilization is sufficiently advanced.
I think a possible solution is to say the field slightly decreases the strong nuclear force within it, and thus making fusion harder. But I am not sure if such a field will outright kill our colonists or have other consequences.