I have installed aem6.5 on my local machine.
On opening the author instances navigation panel is missing only TOOL bar is present.
Which service pack is installed on AEM 6.5?
Do you see this issue on all browsers or any specifc browser? Are there any console errors you see when you try to load the start.html page?
Was this issue reproducible on a fresh vanila 6.5 instance or this happened after installing any specific code or content package?
Please provide screenshot with more details on the issue and we can find out a way to debug.