Greetings Prashant,
Indeed, it is not possible to unlink the Notification from a Work Order in CLSD status, and it is not possible to re-open an Order that was marked as Not Completed NCMP. So this seems a dead end.
Perhaps you could make some changes through BS22 to the CLSD system status control to allow this, but I'm not certain of that at all,and it seems it would be a lot of hassle just to handle this one particular case.
I would propose to your Client that they just create a new Notification, using the original Notification as Reference in IW21 transaction, on the initial screen. Then this linkage is maintained between the related objects...