USB external Hard Drive not being detected by windows

When I came home after work today and used my computer, my Seagate 100 GB Portable External Hard Drive suddenly got an error that says "Operating System Not Found". I am in a big trouble here because I have a lot of important files on it.
I tried a few troubleshooting steps like rebooting the PC, unplugged plus re-plugged the external HDD and even hooking it up to another computer. Windows 7 is my operating system and nothing has changed. Once the external HDD is unplugged, I can boot it to the operating system.
Anyone who has a good suggestion please help me.
Thanks.
