General:
* Idle: Inactive or not in use.
* Inoperative: Not functioning or working.
* Defunct: No longer existing or functioning.
* Down: Not working, often referring to a machine or system.
* Out of order: Not working properly.
* Broken: Damaged and not working.
* Nonfunctional: Not capable of working.
* Disabled: Made incapable of working.
* Inactive: Not currently in use or operation.
Regarding Employment:
* Unemployed: Without a job.
* Jobless: Without a job.
* Out of work: No longer employed.
* Retired: Stopped working permanently, usually after reaching a certain age.
* Layoff: Temporary or permanent dismissal from a job.
* Fired: Dismissed from a job, usually for poor performance.
* Quit: Voluntarily left a job.
Regarding Effort:
* Ineffective: Not producing the desired results.
* Unproductive: Not producing anything of value.
* Futile: Pointless, not likely to succeed.
* Useless: Having no value or purpose.
Other:
* Dormant: Inactive, but capable of becoming active again.
* Stalled: Stopped, unable to proceed.
The best word for "not working" will depend on the specific context.