I met a problem when I were running a routine in Command Line Window.
It closed the ArcMap. After then when I open ArcToolbox, everything gets empty.
See attached pic.

Many effort to figure it out by looking through Internet. Solution is rather simple.
Go to the register, navigate to ESRI fold under HKEY_CURRENT_USER/Software
then rename ESRI to ESRI_old. (Actually you can delete it safely.)

Now they get back.