Lost File system – nothing on C:

After defrag of C:, (successful)everything’s gone: no NTFS, no Windows. Cannot repair (nothing to repair). Like it’s evaporated. No choice but to do fresh install (WinXP Pro SP2), and lose everything (60Gb).
Have seen this before, but that time blamed Norton GoBack not updated to work with SP” – but no Norton this time, clean XP Pro, no viruses, no spyware. No hardware error. Just – phoom, all gone. No idea how to ‘fix’ this, can rebuild, but why did it happen? Anything one can do in such circs?