If you don't its because the bar has been disabled
Looks like this is what has happened
( 'twasn't me, Guv, honest !! - at least not that I'm aware of)
Stop Press !!! ( later )
Fixed.
I wrote Sidebar into the search box of the Start menu, and a heading "Windows Sidebar" appeared as the first file/folder/thingy at the top. I clicked on that and all was restored.
(Didn't tell me why it had disappeared tho' ? )
Thank you.