Recovering files on without formatting

Hello,
I have a 300 gig A-data external drives which was working perfectly before a few minutes ago. all of a sudden it stopped working. i check disk management and the file system shows up as RAW, free space is 298 gig and i mos def have tons of data stored on it. it keeps saying when i should plug it in that i have to format it and that is not an option.
pls help me asap... thanks