My external hard drive won't show up?

My seagate freeagent 1tb external hard drive won't show up on my Mac. It was working perfectly fine yesterday but now it won't show up. I have Windows partitioned on my Mac and it works fine in Windows. But when I tried to connect the drive to a different windows computer it wouldn't show up in My Computer and I couldn't access the files. The drive still lights up and sounds like it is working fine so I don't know what the problem is. Any help would be appreciated. Thanks =)