It wasn't clear in the original post which Operating System was used to do the initial formatting. If it was formatted on a Mac it's probably HFS+ or HFSX. Windows won't recognize HFS+ or HFSX.
If it was formatted in Windows and then used on the Mac, was it properly unmounted before it was unplugged from the Mac? If it wasn't properly unmounted, that could cause some problems. You can download try a freeware tool like TestDisk
to analyze and attempt to fix it.