Can't find files -Help!

Hello,

I have XP and other than my 2 other HD's I have a HD connected via a USB
external box.

It has been fine for 2 years but now for some reason whilst I can see
all my directories I can't see any files in them.

The files I can see in the dirs, which by the way are the ones toward
the end of the alphabet scale, are corrupted.

The disk still shows 11.GB used.

All other drives are fine, I do not have a virus, have UPS so no power
drops and have a firewall through a LAN and gateway.

Any ideas how this happened and how I can retrieve, if possible?

Thanks,

Rock