IPhone 5 :: Some Photos And Videos Disappeared After Certain Date

Before I did importing (from iPhone to computer), everything was fine - all photos/videos were right there on my iPhone. I simply used "cut" - "paste" from Windows File Explorer, and then I found that all the photos and videos after certain date were all gone. They are not in the iPhone, neither in my computer. Luckily all the recent photos are automatically kept in "Photo Stream", so I managed to save them all to the "Camera Roll" and then I could still import them to the computer. However "Photo Stream" does not save the videos, so now looks like I lost those precious videos forever!!  There is another weird thing: after importing, some video files still remained on my iPhone which are unplayable, and the notification said "the requested URL was not found on this server". To ensure I don't lose anything, I still moved them to the computer. And the result is predictable: those video files cannot be played on the computer either, regardless by which player I am trying. I think I am not the only one here. I see another post iPhone 5 some photos and videos disappeared from a particular photo onwards and three videos left are not able to be played and are saved as 0:00 files but I cannot reply.

Info:
iPhone 5, iOS 7.1.2