Once you have added a server to XenCenter, it remains accessible in the Resources pane throughout the current XenCenter session regardless of the server's status - connected or disconnected, running normally or in Maintenance Mode.
To reconnect to a managed server that is disconnected, simply double-click on it in the Resources pane, or right-click and then click Connect on the shortcut menu. Server connection information is remembered for the duration of the current XenCenter session and so you will not need to enter the same login credentials more than once in the same XenCenter session if you want to reconnect using the same user account.
You can also reconnect to a connected server using different login credentials, for example, using your AD login instead of your local root account.