On double click drive is asking for formating

Hello,
I have very serious problem and this is the second time this is happening first time I luckily took the backup from my drive so i had to format as it said. When I started my pc today I tried to open my D drive which is simple drive (which means it is not a window drive). now when i double clicked the drive It asked me for formatting than i tried it opening with right click -> open option which also asked for the same thing. I have got very important data Can you tell me the way to save that data and one thing more it do not access the drive from command prompt as well.

Help me please