USB 500GB NOT OPENING

My external USB hard Drive(SATA) has stopped opening and each time I try it asks me to format it first and when I say yes it gives me the conversation that fomating was not completed because the disk is not accessible.

I am using windows 7 but I have tried to open it with friends who have Vista and XP but it behaves the same way.

So anybody with a solution on how I can recover my Hard Drive please help.