unrecognized external hd

I have a dell laptop inspiron B130 with windows XP Pro. My question is this.
About 2 months ago I unplugged my WD external hd before it was safe to do so.
Now my computer doesn't recognize it in MY COMPUTER and in device manager
everything associated with the usb has an exclamation point in a yellow circle next
to it. What is this trying to tell me if anything?
I have tried my hd in another computer and it works fine. I do not have another
usb device to see if the outcome would be different. I have tried the other usb
ports with the same results and also restarted the computer with the hd plugged
with no change. I realize that this could be fixed by doing a reinstall but I want to
learn how to fix this on my own. By joining SysChat I want to learn as much as I can
about being more proficient at fixing mine and my sons computers. Also I did not
receive an install disc when I bought my computer and I can't locate a reinstall partition
on the c drive I will end up buying one eventually, but that's not what I am trying to do
here. I really need and look forward to any help you guys could give me. Thanks Glenn