Is your external drive still not working?
I'd switch off the drive and unplug it.
Restart your Mac.
After your Mac is fully started, don't open any applications.
Plug in your hard drive, then switch it on.
If it doesn't mount in about five minutes (it may not mount as quickly since it was removed improperly), switch the drive off for 10 seconds after you hear the drive become quiet. (My Lacie's power/disk access light will stop blinking when it's safe to flip the power switch.) After 10 seconds switch it back on.
If it still hasn't mounted in five minutes, check to see if Disk Utility will mount it, if you haven't another drive utility on the drive.
If it still hasn't mounted try another restart of your Mac with the drive plugged in and powered on.
If it still won't mount, I'd suspect the drive was damaged. Do you see the disk activity light blink when you first start up the external hard drive?