Along with vacunita's post, if you don't necessarily need the files but just need the drive then you can format it as he suggested. If you need crucial data, then go with a data recovery program as mentioned elsewhere in the forum. I have used GetDataBack many times with good results except for USB drives.
If you don't want an extra cost associated with this, but still want to recover the data, you can try one more thing. I wrote a recent post regarding a freeware utility that I found that will discover existing partitions that may be corrupt and then will rewrite the needed information back to the drive. Reference thread751-1382280 This has a chance to make things worse if it writes an incorrect partition table, but so far I've had great success with some external USB drives that were seen in Device Manager & Disk Management, but were not partitioned according to Windows.
Good luck,
Mike