There are lots of suggestions online if you google 'start menu missing from Windows 10' or 'calendar missing from windows 10'.
I followed the poster Liam Hughes' suggestion down the page here.... http://forums.windowscentral.com/ask-question/372178-start-menu-not-working-windows-10-after-upgrading.html
I have Avast installed as my antivirus.
When I followed the instructions, then restarted my computer, my start menu appeared, and a calendar appeared in the bottom r/h side of the screen.
Everything seems to be ok now, but why can't Microsoft get these things sorted beforehand? Seems like so many people have the same problem.