General Antonyms:
* Shortened
* Abbreviated
* Curtailed
* Brief
* Concise
* Short
* Limited
* Rapid
* Quick
More Specific Antonyms:
* Shortened (if referring to duration)
* Stopped (if referring to an action)
* Abrupt (if referring to a sudden end)
* Hastened (if referring to a quickening of pace)
Example:
* Prolonged: The meeting was prolonged for hours.
* Antonym: The meeting was abruptly ended.
The best antonym for "prolonged" will depend on the specific situation and what you are trying to express.