Currently it is not possible to re-enable under certain conditions a closed task inside the playbook designer.
However some playbooks require to ask analyst few questions and depending on his answers to go back to a certain previous step inside the designed process, like "have you successfully patched the workstation?" and if he replies no, then the workflow shall loop back to a previous step like "download the patch" or whatever. Or in ASCII-art I mean:
START ---> STEP 1 ---> STEP 2 ---> Question : Y/N ? ---> Y: END
|------------- N: back to step 1 -|
In this case <STEP 1> would be a "Conditionaly reactivabe task linked to result of the Question (here: "No", but it could be a value, any type already supported).