Unallocated drive?

[exe163]

I have a Western Digital 500GB external harddrive, formatted FAT32, been using it for few months not a single problem. But after using it to back up some files from a dead laptop via ubuntu, the harddrive stops functioning properly. Under Computer Management, I can see the drive in black with no drive letter, only showing:

465.76GB
Unallocated

What is this suppose to mean? And is there anyway to fix it without reformatting the drive, there are some file inside I would like to keep.