Do you hear the drive starting up at all? Do any lights come on?Does it make a clicking sound? Does the hardware show up in the system device manager?Also try connecting the drive with another cable.
I am also having the same problem-"all of a sudden, my seagate 500 Gb external drive is not working". I have tried all the possible ways from my end-changed the cable, tried in another laptop......no sound, no show up in the system device manager..It doesn't starting up at all. ANY HELP WILL BE MUCH APPRECIATED...!!!