Capacity Planner should present a dialog box after extended periods of inactivity to provide for the option to log out or continue using the product. In some cases, this box does not appear. If a timeseries does not appear when you attempt to open it, it is likely that the UI has logged out automatically due to a prolonged period of inactivity. When this happens, reload the tab. You will be taken to the login screen.
Once you have logged in you can continue as normal.