You can open a Workflow Interactive Task for an existing Workflow Job by passing the task id into workflow.openTaskAndGotoForm(taskId). This is usually done in a Tasklist.
This assigns the task to the current user and navigates to the task's associated Form. The Form receives Workflow in parameter values from mapped process attributes.
Open tasks can then be completed to continue the workflow process, or if the session has ended, can be reopened by the same user using workflow.reopenTaskAndGotoForm(taskId).