HI awswin. Connect your hard drive. do you hear the "tutin"(hehe) sound when you connect USB devices? If you dont, that means you have a serious problem with your hard drive. Either the case is bad so you just remove the hard drive from the casing and put in on another one (it could be any extrenal case for hard drives.. as long it has the same interface - SATA or IDE). If you do hear the "tutin"sound, go to Start - right click on MY computer and click on "Manage". Then go to "Disk Administration" above "services and applications" and verify if you see your hard drive. Probably you have to initialize the disk or Assign a letter by right clicking on it. But first try removing the hard drive and connect it to another computer WITHOUT the case to make sure it's not the hard drive. I hope this helps and dont forget to rate my answer if it did. Thank you :D

Can be quite a few causes. Have a look at this article on troubleshooting external hard disk problems, maybe it has a fix for you.