Hidden or collapsed?  If you are referring to the menu bar on the left, it doesn't hide itself unless you click on the margin to trigger it.  If you're referring to the navigation tree, the default is collapsed unless you click on a section to expand it.

If these are not working as described, you may want to check that you're using a ProConnect Tax-supported browser and make sure open it only in incognito mode, always, which helps avoid most common issues.