Interesting. I don't think setting job_queue_processes from 0 to 1 had anything to do with it. It's got to be something else. I'd be curious to know if it will run now if you set job_queue_processes back?

Do you have snapshots or materialized views?

JOB_QUEUE_PROCESSES specifies the maximum number of processes that can be created for the execution of jobs. It specifies the number of job queue processes per instance (J000, ... J999). Replication uses job queues for data refreshes. Advanced queuing uses job queues for message propagation. You can create user job requests through the DBMS_JOB package.

Some job queue requests are created automatically. An example is refresh support for materialized views. If you wish to have your materialized views updated automatically, you must set JOB_QUEUE_PROCESSES to a value of one or higher.