All windows, e.g. Workbench, Explorer, Properties, can be hidden from the graphical interface (by clicking on the cross at the upper-right hand corner of the window, for example). To make them appear again, click VIEW->Workbench in the main menu of Sim4Life.
Note that since the windows can be resized and moved around, in some cases, the Workbench window can be barely visible, or even detached from Sim4Life, possibly even on another monitor. If the window appears to be truly lost, one can reset the GUI appearance to its default configuration.
It is located in: This PC -> Documents -> Sim4Life -> 3.4 -> Preferences -> UI.h5
This PC -> Documents -> Sim4Life -> 3.4 -> Preferences -> UI.h5
To do so, please launch the Registry Editor by typing "regedit" in the Windows Run command line. Then Navigate to HKEY_CURRENT_USER\SOFTWARE\Schmid & Partner Engineering AG\Sim4Life\3.4\SuperMash\Settings and delete the entire Settings folder. This forces Sim4Life do rearrange the dockable windows in default configuration and automatically re-write this registry entry.
HKEY_CURRENT_USER\SOFTWARE\Schmid & Partner Engineering AG\Sim4Life\3.4\SuperMash\Settings