Cubase 6 video "incorrect format"

Hey, there is something really bizarre happening over and over again:
I am working with some video regularly, for hours, and suddenly Cubase 6 crashes…when i open again, the same video that was working normal, dissapear from the timeline and in the Pool it says its a incorrect video format!!!
How is this possible, since i was already working for hours/days with no problem with THE SAME VIDEO?

Anyone with similar problem?
This is driving me crazy!!!