Quit TBC
Go to the folder
C:\Users\ASHARP\Appdata\ (put in your username where ASHARP is of course
Then select the following path
C:\Users\asharp\AppData\Roaming\Trimble\Trimble Business Center\31.0 (you may not be 31.0 (I am using a daily build)
Sort the contents of the folder by Date and Time (Newest on top)
You should find a File with the extension .Layoutd (note the d in Layoutd as there is also a .Layout file
If you delete the .Layoutd file and then restart the TBC application then the defaults for Pane Layout will be restored.
If you delete the .Layout file and then restart TBC the defaults for Menus and Toolbars will be reset to defaults