How can I get back my data in my external hard disk drive?

I have an external hard disk drive. It has a 150 GB space. The disk is 77.7 GB full with movies & very important documents. Yesterday, all things were working OK, but today, when I insert the disk, my laptop gets hung. The disk is being detected, but after opening, it is not getting open up. It shows a pop up message” You can use your disk after formatting it”. I have tried to scan it by antivirus but it’s not getting scanned. I want to save my data. The disk’s name I had given was “Transcend”. Today, It is not showing the name “local disk”.
How can I recover my Hard disk drive? I wonder, How it has broken down all of a sudden?.
Is there any way to save my data? I don’t want to format it. How can I get my data? Is there any software to repair my hard disk?