Try right clicking, then "open with" then "choose default program" (if you see a submenu to the right/left) or "more" (if you see a middle of screen pop-up instead of a side submenu), then choose the option to look for a program, navigate to the folder in which notepad++ has it's executable file, select this and open, notepad++ should now be default or at the very least will be visible in the context menu next time you right click on a file of the type you have just opened with it.
P.S. Just a quick check, you've probably got 64 bit windows, but you might have been running 32 bit notepad++, if so then try 64 bit notepad++ instead, because sometimes 32 bit programs on 64 bit windows don't show up in context menu so well (I had it with 7z once).
Edited by rp88, 20 November 2017 - 06:41 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB