If you close the initial dialog (where you can connect to a ROS server) you cannot get it back.

Several releases back, the window menu had a way to re-open the initial dialog but that has disappeared. Right now the only way to connect to a new ROS server is to quit studio and re-open it. (Note - the ROS Open dialog is there in the window menu until you close it ... then it's gone.)

Pleas put access to the window back into the Windows menu or add a "Connect to ROS server..." option in the file menu.