It appears you have a major file system corruption. This is not an Excel question but a Windows recovery issue. You may need to use utilities such as CHKDSK or similar to see whether it can rebuild the file system. Then, you still may need to recover these specific files from backup. I wouldn't trust anything on that server until this is all done.
Oh, and get a proper battery backup in place and make sure to use its usb connection so Windows knows to shut itself down safely before the battery is dead.