Well that sucks, here this should work for you(as long as time machine doesn't interfere). You did say it still shows up on the Mac so we will use its utility to erase it back to FAT32.
I just found some nice pictured instructions(better than me typing it out by hand) at:
Formatting an External Device - Mac Guides
When it comes to selecting the type you want to format it as, make sure to select MS-DOS File System. After this is done you should be able to get it back up and running on your windows machine as this is a nice wonderful format that windows can use lol.