If you're thinking of "good fortune":
* Misfortune
* Bad luck
* Unluckiness
* Hardship
* Disaster
If you're thinking of "fortune" as in wealth:
* Poverty
* Indigence
* Destitution
* Want
If you're thinking of "fortune" as in destiny:
* Free will
* Choice
* Control
It's important to consider the specific meaning of "fortune" in the context you're using it to find the best opposite word.