You have two options: Use the Alt-F11 shortcut or using the system menu of the application you can check the "Top Pane" command. Please see Top Pane for more information.
The navigation pane can be displayed in many modes. You can select the tree view with View - All Entries - Tree View or use the default shortcut F7
View - All Entries Menu
Use the Reset Layout option to revert back to the default UI layout in the Window tab.
Reset Layout Button
The reset layout might not resolve all the issues. Close the application and delete the layout files (*.lyt) in %LocalAppData%\Devolutions\RemoteDesktopManager to completely restore the layout.
Quick Access Toolbar
To reset the quick access toolbar, close the application and delete the file remotedesktopmanager.qtb in %LocalAppData%\Devolutions\RemoteDesktopManager
Only your favorites are displayed even though you are in the "All Entries" tab, the root node is also entitled "Sessions - Favorites Only"
You may have inadvertently enabled a filter. Click on the ellipsis button of the search filter box and deselect the Show Only Favorites menu item.