Only Sami can give a definite answer, but UrW isn't as good at handling crashes as one might want (although Sami took some steps lately, which I think is for the next update). UrW writes things to file at check points, and in addition to that the OS may decide on its own that it won't actually commit the written data to disk for some time, and if the power goes at that time what was committed but not written is obviously lost. The OS might not like incorrectly terminated files either (as happens when the application gets killed before it has closed the files).
If Sami says he can try to help you, it would normally be the complete folder for that character that should be zipped up.
The standard recommendation for UrW is to make regular backups of the character folder (I typically copy it after every sitting) in case things go horribly wrong. Obviously, that doesn't help you after the fact, though.