Why has Windows 10 stopped behaving normally?

I am running Widows 10 on a Lenovo G500, having upgraded months ago from Windows 8.

Since the most recent upgrade, a few days ago, the button in the bottom far left hand corner has become completely unresponsive. Previously, clicking here would give me a range of options including settings and power, which I used to Sleep or Shut Down. Now I can only shut down by pressing the physical on-off button on the computer, and this seems to just be taking me to Sleep mode. There is no way I can find the full startup menu with settings etc.

What do I need to do?

Read more: Why has Windows 10 stopped behaving normally?