I lost my bitcoin wallet, I have the key but lost the wallet, it was saved on an external hard drive but was stolen from me,
how I can get my wallet back?
the second question, If someone have bitcoin and have the bitcoin address, but lost the wallet, can the bitcoin be retrieved? and transfer to another wallet?
If you lost your external hard drive that has your wallet file inside, it's big risk to lose your bitcoin.
I believe you actually know addresses in that wallet, with your bitcoin stored there, check these addresses on Bitcoin block explorer.
List of useful Bitcoin block explorersIf you see balance is 0, you already lost your bitcoin. Nothing more to do.
If balance is not 0, and your bitcoin was not moved to new addresses, you must do a next step as soon as possible. Import your wallet, recover it with private key or mnemonic seed and move it to a new wallet. Send your bitcoin to a new address in a new wallet, because you must assume that old wallet already lost, someone will have access to it and can steal your bitcoin.
Restoring your standard wallet from seedSweeping your private keys into Electrum