If one of the creatures instructed to fight is no longer on the battlefield, what happens?

When a fight occurs in Magic: The Gathering, the effect instructs two creatures to deal damage to each other equal to their power. If one of the creatures that was instructed to fight is no longer on the battlefield at the time the fight effect resolves, the other creature involved in the fight does not deal any damage.

This happens because the fight effect requires both creatures to be present to deal damage to one another. If one creature has left the battlefield, the mechanic of the fight cannot be completed as intended, and thus, no damage is dealt at all. The rules governing such interactions ensure that a creature that can't engage in a fight doesn't result in any unintended harm or damage dealt under those circumstances.

Overall, the fight mechanic specifically relies on both creatures being available to deal damage simultaneously, making the situation where one creature is missing result in no damage being dealt.