Hello!
After a blackout, something happened to a friend of mine's hard disk. The BIOS doesn't list it anymore among bootable devices, and i get a boot failure message at startup. I tried booting the disk on another computer, and still get the same result. It seems quite clear the the hard disk is damaged, but i was wondering if there is a way to recover its content.
Any suggestion?
Thanks!

It's dead Jim.

First, always make backups.
Second, there are some companies that do disk data recovery.
This is expensive, and no garantuee on recovering everything.
Third, a cheap way to prevent this for the future would be a 3disk raid5 setup
Choose hot-swappable technology, so a damaged disk can be replaced easily.

First, you have to check if the BIOS understands that a disk is connected. To be bootable or not bootable is a minor issue. You can boot from a Live CD and test the disk or even backup files to another device if it is possible.

Check if your friend's motherboard controllers are in good condition after the blackout.

Finally, if the mechanical part is in good condition, it is perhaps possible to take the electronic card from another (same model) disk and put it in the place of the damaged card. I don't know how this is easy to do, obviously in a service laboratory.

This doesn't appear to apply in your case, since you removed the drive and tried it in another machine. But I recently experienced the same thing. We lost power for a few hours and when it came back, one of my machines wouldn't recognize the drives.

Sweat beading profusely on my forehead, I turned off the power switch on the back of the power supply. Switching it back on and hitting the start button, the machine came back up.

I'm not exactly sure why this would make a difference, but apparently it did.

Somestimes a complete power off/on helps.

I have even seen Disks repairing themselves over time. (1 week)
While a FRU had already been ordered.

wow....
I want one of those... the hell one! Give me twenty of those, pronto!

IBM x3850-CTO
Raid 5 setup with 3x 2.5" SAS

Thanks to everyone.

I did try with the live cd (slax), but the hard disk didn't show up.

I really don't know what to do.. Maybe i'll check it again in a week :-)

I had a drive that was reported by the BIOS as having SMART status of BAD (it was even in capitals! scary...). I recently acquired a USB 2.0 Hard drive enclosure. I plugged in the BAD drive, and it was able to be formatted perfectly easily that way. Admittedly my case differs from yours in that my BIOS could still see the drive, but it might be worth a shot. Having said that, I didn't get anything useful off it, but I did manage to format it and use it for a while. After a couple of hours though, really strange things started happening, and I got a dmesg full of errors, so I binned it. Perhaps yours would fare better?

Otherwise there's the classic put-it-in-a-bag-in-the-freezer trick.