Recovering funds after I lost my wallet.dat I have received the bitcoin way back in 2013 in Bitcoin Core.
At that time I don't know about the importance of wallet.dat - I have formatted my hard disk and install Bitcoin Core again in 2017 but there it didn't show my Bitcoin in my balance. I have my Bitcoin Key.

How can I get my bitcoin back? Please help me!
If you have your private key, you can add it to your wallet with the command
bitcoin-cli importprivkey "&lt;YOUR_PRIVATE_KEY&gt;" "" false

(The false at the end is to cancel rescan at the moment of adding. It could take some time.) 

If you do not have the private key associated with the address the coins were sent to back then, they are lost.