My first thought is that it's been dropped
What sort of caddy is it - USB or Firewire - can you try plugging it into a different socket, also try plugging it into a different computer, perhaps at work.
If none of them recognise it, then the final thing you can do is remove the drive from the enclosure & temporarily plug it in to the motherboard, to make sure it's not the enclosure playing up
Are you running XP ?