Problem in the WD

I have a problem in my WD Drive,  suddenly when I make a connect between the hard desk & the computer,  the hard desk didn’t work at all… I Purchased it in April 2009 , it’s work with me UP until yesterday but it stopped now…
PLEASE Help me

information about it :-
WD P/N: WD5000MES - 00 2809A

By the way I saw a program when I Purchased it but I didn’t run it because the hard desk seems to work even without it
I’m sorry my english is bad but I hope you understand something…

The computer doesn’t recognize the hard desk’s existence