"I want to make my hard disk Drive readable and recover my data."

I use two HDDs for my PC. In one am using Ubuntu11.04 os. In another HDD, I was using windows. With the help of Ubuntu, I have tried to format one of the free partition of the hard disk drive, from which it was having windows O.S. But as soon as I tried, without getting any prompt, accidentally the drive became "unknown". What Can I do, to retrieve my data? Is there any command like unformat? Or any software or tool to get back my important data?
Please help!