Recently, I was using a 3rd party product that had toolbox items.  All was good after installation, then Visual Studio crashed, and my new toolbox items disappeared.  Restarting didn’t help.  I came across the following solution which did solve my issue. To understand the cause of Toolbox issues and resolve them, perform the following steps:

• Open the "%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\14.0" folder and remove all .TBD files.
• Run the Registry Editor (regedit.exe).
• Locate the following keys: "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\14.0\ToolboxControlsInstaller_AssemblyFoldersExCache" "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\14.0\ToolboxControlsInstallerCache"
• Remove these keys with their sub-keys.
• Launch Visual Studio 2014 using the following command line:

In a 32-bit Windows: "C:\Program Files\Microsoft Visual Studio 14.0\Common7\IDE\devenv" /ResetSkipPkgs /log "<my_folder>\ActivityLog.xml" In a 64-bit Windows: "C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv" /ResetSkipPkgs /log "<my_folder>\ActivityLog.xml"

• Open Visual Studio – relaunch and hopefully all is good.