sounds like its bust tbh. the hard drive heads are not moving correctly or getting stuck, hense the clicking sound. iv had this several times and theirs a few temperary solutions but ultimatley your disk is going to die, so if you get it working, BACK IT UP ASAP.
solution one: try turn the HDD upside down, turn it on and see if it still clicks, if not then its working. if that doesnt work, switch it off, turn it on its side, etc etc etc iv had this work for me several times.
solution 2: wrap it in a plastic bag or cling film and place it in the freezer over night. it its a stuck head then freezing it will contract the metal head just enoight to free it and you might get it working long enough to get any important info off it.
i dont think theirs any perminant solution to fix your problem but one of the above solutions should work and give you time to back it up.