Did you know... How to hide or show the solution in solution explorer? - #156
Sometimes, you might find yourself in a state where the solution is not shown. For example, some of the default environment settings have this enabled, like the Visual Basic Development Settings.
If you find yourself in this state and want the solution back (or if you want to hide the solution), go to Tools – Options – Projects and Solutions – General and check Always Show Solution
Have said all of this, VS will override this setting and always show the solution if the solution has 2 or more projects in it.