Notes from Github:

In a public repository, scheduled workflows are automatically disabled when no repository activity has occurred in 60 days. For information on re-enabling a disabled workflow, see "Disabling and enabling a workflow."

When the last user to commit to the cron schedule of a workflow is removed from the organization, the scheduled workflow will be disabled. If a user with write permissions to the repository makes a commit that changes the cron schedule, the scheduled workflow will be reactivated. Note that, in this situation, the workflow is not reactivated by any change to the workflow file; you must alter the cron value and commit this change.