I've been running 4 copies of Windows 10 since 10/1/14 and haven't seen this myself, but I have seen others report it. You might want to take a look at this link: http://answers.microsoft.com/en-us/search/search?SearchTerm=start+button+disappearing&IsSuggestedTerm=false&tab=&x=0&y=0&CurrentScope.ForumName=insider&CurrentScope.Filter=insider_wintp-insiderplat_pc&ContentTypeScope=#/insider/insider_wintp-insiderplat_pc//1
I have been using Windows 10 for three months. At least once per week I lose the start menu. I go through the routine of safe booting etc. which temporarily fixes the problem. However, I have noticed that in the Boot menu it states that my current and default OS is Windows 8.1. Did Windows 10 not upgrade properly? Any one else having this problem?