If an upgrade to vCenter Server with external Platform Services Controller fails, you must manually restore or repoint vCenter Inventory Service or other vCenter Server services.
If a vCenter Server upgrade failure occurs after the uninstallation phase and reverts the setup to the previous state (vCenter Server 5.5), it might not reregister vCenter Inventory Service or other vCenter Server services with the vCenter Single Sign-On included in Platform Services Controller 6.5.
vCenter Inventory Service and other vCenter Server services are unregistered from vCenter Single-Sign-On 5.5 during the upgrade to vCenter Server 6.5. If an upgrade failure occurs after the services are unregistered, the registration information is lost. When the upgrade to vCenter Server 6.5 is resumed, the installer sees unregistered services and leaves them unregistered. The vCenter Inventory Service or other vCenter Server services must be manually repointed or registered with the newly upgraded Platform Services Controller 6.5 instance. See Knowledge Base article 2033620.