We dont know why QVD file got corrupted. Generally, while saving the data QVD it take some time depend on the size. on particular instance time, Qlikview took lot of system resources depends on the file size. If the server not avaliable at that time we may loose some data.
May be you need to come with some other QVD storage strategy to avoid this. It better break the QVD into Quarterly or Monthly. Even though if you loose the data, recovery won't take much time