Lost files on WD Firewire Hard Drive when moving from MAC to PC

I was using my Western Digital firewire external HD on my Mac. Unplugged and transferred to my PC. Installed driver on PC but do not think I reformatted. Recognized HD with the wrong name and none of the data was visible. When I took it back to the Mac, the original name was gone (now it is NO NAME) and all of the data is not visible either. It says I have 111GB avail and it is a 120GB drive.
Can you help me??