When someone losts his bitcoin wallet he/she lost his bitcoins and never possible to get it but where goes it? or Can anyone get it or be benefited like traditional currency?
Gone forever. It's the greatest tragedy of Bitcoin. Perhaps second only to the rest of us not knowing how many bitcoin have been lost.
There's rarely a way to get your coins back. Now if you forget a password but know the public keys of your wallet, there's a chance you can crack the password. But if you've also lost the public keys and don't have the wallet client on your laptop or something those coins are lost forever.
Keep your password and keys safe!
The greatest thing about Bitcoin is that we all get to be our own bank. And the scariest thing about Bitcoin is that we all get to be our own bank.