"The power supply to the pc was switched off while the HD was running"
Er - every time you switch off a PC, the power is switched off the hard drive - that's how it stops running! There is no power switch on a hard drive, it turns off when the power to it is turned off.
Either the drive has failed - in which case you've lost what is on it unless you take a fairly expensive specialist data recovery route.
Or the drive is corrupted. In which case the USB connection to a working system route, plus recovery software, can be tried.
Of course you need a working system to try it.
If the BIOS cannot even detect the hard drive, then your chances of being able to run a recovery program on it are slim to nothing. However if the power went off the system in a power cut or power surge situation, it's also possible that something else has failed (power supply, motherboard, etc) which is preventing the system from finding the drive.