I misplaced my bitcoin wallet. I still have the key, but the actual wallet was on an external hard drive that got stolen. Is there any way to recover my wallet?
Also, if someone has a bitcoin address but lost access to their wallet, can they still recover the bitcoin and move it to a different wallet?
Keep in mind that sweeping creates a transaction to transfer the coins to the wallet you (just) created. This means you pay the miners' fee. And this means you have to keep super safe the seed of this new wallet.
If you want to just recover your wallet (and you're sure the private key was kept safe) you can just restore your wallet in Electrum on the "use public or private keys" path.
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 explorers
If 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 seed
Sweeping your private keys into Electrum
I hope you did not keep your keys in that hard drive, if the thief knows that it is where you stored your wallet and the keys then time is of the essence that you download or create a new wallet since you have the keys you can sweep it and transfer to a new address.
I had a similar experience when my hard drive was lost and being a newbie then I kept my private keys to the lost hard drive.