i dont know if anyone had posted the solution for this before, it has happened to me a couple of times, when the first bar of the conversion is 100% and the second stops on 49%, but i found a way for not lose the video.
when this happened the video is already converted, you just have to find the folder that contains the video, and the tumbnail of the video, in my case it is:
C:\Documents and Settings\username\my documents\Sansa Media Converter
then drag and drop the video on your fuze.
sorry for my bad english, it sucks.