In general, its always better to umount your USB drive before removing it. I'm suprised to hear your drive is completely unrecognized by the OS now -- this typically only happens if the partition table was being written to while it was unplugged (a very rare occurence). You may want to show us a copy of your console messages when you plug the drive in. If the drive is recognized as an HFS+ partition, the OS will automatically attempt an fsck process which will do a very high level recovery but it sounds as if your drive is not even being recognized.
What exactly is the error you're getting when you plug it in?
Have you tried power cycling your Mac and your drive?
Here is a silly one: Make sure your power supply is plugged in fully, and that you're not using a different PS than the one shipped with your device -- many of them look similar these days, but don't actually supply the same power.
Do you happen to have backups of your data? If so, you should feel easier about using some of the disk utilities available with the OS and online for purchase.
If not, you need to proceed very carefully -- even experienced technicians can wipe out your data in the process of recovery, and if the data is very important to you, you may want to consider sending it in to a professional data recovery service -- these can be expensive, so be careful.
There are utilities out there which will scan your drive for file fragments, and attept to reconstruct what may have been corrupted, but I don't yet have experience using HFS+ recovery tools, so I can't advise you.
Golan.
