Damn. Why can hacker steal your assets? Are you surfing on an unsafe website? Or your mistake that you input private key in the form airdrop/bounty?

Yes, you can always secure your wallet and your assets. But there is something you need to pay attention:
- If indeed you have been surfing on an unsafe website, you need to re-install your device, upgrade your antivirus, I advise you to upgrade to Antivirus  Pro. Maybe your device has infected some viruses and possibility the hacker steal all of your data.
- After the first task done, you just install MetaMask at https://metamask.io , and save the mnemonic phrase from MetaMask to the safest place!
- Or You can to buy a Ledger Wallet or Trezor, it's more secure to safe your assets.