I can't tell you why you lost your file system; But if you have saved regularly your .vfs file, you will find in the directory where you had saved your file system, one or some *.bak files.
He'll just rename this / these file(s) by changing the extension .bak in the extension .vfs and reload the file with HFS.
You keep the one that seems more up to date.
Otherwise the general rule for avoid this is to always keep in a safe place, a recent copy of hfs.ini file and of your file system *.vfs