Hey there, Descartes.
I'll need a bit more info on the drive in order to figure out the best course of action. First and foremost do you need to recover any data from that HDD? What do you mean by "hard disk not responding" exactly? Is it that the drive is not recognized anywhere; or that it doesn't start at all?
The generic external troubleshooting steps are to try the drive with a different USB cable (shorter than 12") and a different computer, to see if the same thing happens. But from then on it all depends on whether or not you have data to recover and on the state of the HDD. So let me know what is the situation with the drive.
I'm looking forward to your response.
Boogieman_WD