General:
* Fortuitous: Happening by chance, especially in a fortunate way.
* Unforeseen: Not anticipated or predicted.
* Unexpected: Not expected or anticipated.
* Impromptu: Done without being planned or rehearsed.
* Serendipitous: Occurring or discovered by chance in a happy or beneficial way.
* Fateful: Having a significant and often negative impact.
Negative:
* Catastrophe: A sudden and disastrous event.
* Calamity: A serious misfortune.
* Mishap: An unfortunate accident.
* Disaster: A sudden and great misfortune or failure.
* Adversity: A state or instance of misfortune or hardship.
Neutral:
* Circumstance: A condition, fact, or event that contributes to a result.
* Contingency: A possibility or possibility of something happening.
* Eventuality: Something that may occur in the future.
* Incident: An event or occurrence.
Positive:
* Opportunity: A chance for advancement or progress.
* Breakthrough: A sudden, dramatic, and important discovery or development.
* Windfall: A sudden and unexpected piece of good luck.
The best word will depend on the specific context and the tone you want to convey.