When nothing is displayed when restarting a chat, it is likely that you set the initial message to not display when restarting.
Here, you can check the setting with this procedure.
(1) Click the setting button on the upper right of the management page.
(2) Click [Chat]>[Functions]>[Initial message] on the left side of the screen.
(3) Click [Resume].
(4) If the box [State this when offline] is unchecked, the initial message will not be shown when restarting. Change the setting if necessary.