First thing comes to mind is that yes the files are corrupted and have become so when transfered.
The problem will lie in fixing the corruption, Sibelius using a properietary .sib extension which cant open in any other program i know. I would revert to the master copies providing you have them and failing that copy the corrupted files using DOS to your PC and try to open them there. Sometimes miracles happen.
Philbert
August 2007