HDD not showing up

I have a western digital 250 GB External Hardisk which I used to connect to my computer using USB cable . Suddenly it stopped working , when I plug in it gets detected and I can see it in the Device Manager but when I go to My Computer I see nothing . I thought of a partition problem and then I went to Disk Management to assign it a letter and it took few minutes to get it loaded and I saw no HDD mentioned there . 

Please help 

have you tried a firmare update?