Full version of windows 8.1 (64 bit) isn’t showing the start menu

Hi, Recently I bought windows 8.1 full version 64 bit. After installing, it was playing soft. But after a few hours of non-stop working the windows is slowing down and the most disturbing thing is happening that, the start menu isn't showing at all (sometimes). Sometimes restarting windows is helpful but not always. Can anyone solve it?
