You do realise you've posted an external hard disk problem under generic PDA problems?
From the sound of it you disconnected the drive without ejecting/unmounting it, please let us know what operating system you are using so we have some idea what disk repair tools you have available, from the sound of things you are using (suffering?;) Windows but what version.
Do you know what file-system the disk is formatted with? If you have never formatted it or have formatted under Windows without selecting any options it will almost certainly be formatted as FAT32.
If indeed you are suffering Windows you should have had a tools CD supplied with the drive, if you don't have it or cannot find it you will realistically need to download and burn a live Linux CD to provide rescue tools;
if you have a fast internet connection download Ubuntu from http://www.ubuntu.com/getubuntu although this is marked as an installer CD it functions as a live CD first and gives the option to install only if needed. If you do not have a fast internet connection download Puppy linux form here
it's not particularly pretty but it does provide a full set of tools. **don't forget either CD image needs to be burned as an image not just copied to a disk.
Once you have provided the requested information and have a tools/live CD to hand we should be able to help further however you my get more responses if you post thus under the appropriate category.