If you are using the desktop app, then the GoTo MeetingViewer Window might be hidden behind another application running on your computer, or it might be minimized. Check the task bar (Windows) or dock (Mac) for the app icon, then select it to bring the desktop app to the forefront of your screen.

If you are using the web app, then your session takes place entirely within your web browser. This means it's possible that you are looking at the wrong browser tab. Check the tabs that are open (and be sure to check for other browser windows), and select the one that says GoTo and shows the red "audio live" icon .

When no one is currently sharing their screen during a session, the Waiting Room is displayed and you'll see the message Waiting to view <Presenter's> screen. Once someone starts sharing their screen, you will see it in this same window. In the meantime, you should still be able to hear the audio conference and chat with others.