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You should back up your wallet regularly.
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Sorry, how to do that ?

I think blockchain is secure, I don't use pc wallet because i am afraid something happen to my pc.
Blockchain.info is not secure if you access it from a PC that has been infected with malware. The malware on your PC can capture your wallet identifier and your password when you type them in, and then the thief can access your blockchain wallet and steal your bitcoins.
If you don't trust your PC, then you should not be using blockchain.info.