I found the solution. After Restarting the Human Task Server we need to load the Session that created the Task and use below code to register the handler once again with the Session and Connect it before performin the Complete Operation
CommandBasedWSHumanTaskHandler wsHumanTaskHandler=new CommandBasedWSHumanTaskHandler(ksession);
ksession.getWorkItemManager().registerWorkItemHandler("Human Task",wsHumanTaskHandler);wsHumanTaskHandler.connect();