See if a different computer recognizes it. If not, it's possible that the SATA interface card inside the enclosure has failed. The good news is that if that's the problem, you can still save the drive by removing it from that enclosure and putting it in one like this:
Hi, thanks for your answers!
I am trying now on a 32bits Vista laptop and it does recognize it (The system asks me to format it in order to be able to use it)
I´ve trying since the problem occurred, this is the first time asking.
Thanks for any guess and help!
@Turbostar, I have always upgraded Mac software as soon as available, and the hd did work