You must back up your wallet. You can import it on multibit wallet if blockchain ceases to exist or have some service interruption.
What if I did not backed up my wallet? What if that BTC is gone, where can I go or who can I contact?
If you are talking about blockchain.info wallet, I believe they would still send you a backup if they cease to exist. Just contact Blockchain.info support, I'm sure they will help you. They also send you email backups in json format from time to time. At
http://blockchain-status.com/, the server says that if the server gets compromised, they would send you a email backup. I believe it would be the same for a shutdown.
If you are talking about desktop wallet, you can't get your BTC back. To spend your BTC, you need a private key of the address or the coins inside the address cannot be moved. No one else except the person with the private key can transfer the coins in the address.