unable to view external hard drive in my computer

I have Windows Vista installed on my Toshiba laptop. I have recently purchased a new Western Digital 500gb hard drive and it was working fine on this same system without any issues. I was able to copy all the data on this hard drive but after playing for a while with this hard drive I accidentally changed its drive letter and now it has disappeared from my computer. All i get is a little black box with all me data on it and I am not able to access it anymore. Can anyone please help me out. Thanks for any information

Have you tried attaching it on some other computer and checked if it can be seen or appearing there. If it is showing then restart your computer and then connect the drive into a different USB port. Also go into Disk Management and see if the drive is getting listed over there, right click on it and simply initialize it.