Why won't Windows Movie Maker load my videos?

I have been using the same file for months, from my Nikon camera. I checked and it is a recognised file to WMM. I have spent 2 hours filming videos and when I come to load them, the "unrecognised file" icon comes up on the videos…

Why is it doing this and how can I get it to read my files?

Thanks

Added (1). The file is a MOV. File, but it has been accepting it for months, but all of the sudden stopped reading it.

What is the file name?