You probably store your Bitcoins in a wallet. A wallet is nothing more than the storage of two cryptography keys – one public and one private key per Bitcoin.
If you’re keeping your coins on an exchange, you are not in sole control of your keys, and your Bitcoins are subject to loss.
Avoid using online wallets as a primary means of storage.
Encrypt your wallet and remember your password. Forget your password, and your coins are lost forever.