Menu button not bringing settings up. It reduces the windows. How can I enable the menu button?

[AC Question]

Menu button not bringing settings up. It reduces the windows

Hi everybody. I have only today took ownership of an S4, previously using an S3.
I updated the new phone to lollipop 5.1. No problem. (I hope)
I swapped all my contacts, music over etc, No problem. But, since doing all this,

The menu key has stopped working, (It wont bring the settings up etc)
All it does is reduce the main screen, as if I wanted to edit or remove pages.

Can anybody tell me how to enable the menu button again Please

Thanks
Mark