Check the Windows updates is my first suggestion. If not, you can try two other ways:
Ctrl + Alt + Del - bottom right corner, shut down - see if that works.
OR
Start > type 'cmd' and then 'shutdown /s /t 0' (ignoring the inverted commas).
Finally, may I suggest an upgrade to Windows 10? Might cure this problem and you can set it so that either you have the start screen like in win 8 or go back to a start menu ala win 7. And as far as I know, until June 2016, it's free (I think MS is allowing a year of free upgrades). Of course, it's not my place to say, just something to bear in mind!