I was about to reply to a post about the Open With menu disappearing but the post itself disappeared. 
I guess the poster solved the problem by himself and removed it, but just in case someone else wanted to know the solution here is my reply anyway:
Did you turn on the option to hide Windows items on context menus? If so then that's why it's gone. You can either turn the option off again or you can force it to come back by adding this to the Context Menu of the All Files filetype:
Action:
OpenWith_GUID
(It doesn't actually matter what you call it.)
Function:
Standard Function (Opus or external)
Body:
FileType CONTEXTFORCE CONTEXTMENU={09799AFB-AD67-11d1-ABCD-00C04FC30936}