Does it look like this?


If not, can you take a screenshot of what your wallet looks like and upload it on an image hosting site so we can know exactly what wallet you're using?

Edit: note that you can always import your bitcoin wallet seed (a list of 12 words you're given when you install the app) to another bitcoin wallet so you don't have to use that particular one.

The transaction that sent out your bitcoin was made barely seconds after it was received, so most likely there was a script running (probably on your PC) that sent the bitcoin to the attacker's address almost immediately, and also overpaying in fees (88.7sats/byte while the average on that day was ~15 sats per byte).

Like the posters above have said, either your version of electrum is malware, or your computer has been compromised.
Run a complete antivirus scan to find out.

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You'll have to pay a developer with experience in solidity to go through your contract for bugs, etc.
Zeppelin is one of the many companies that audits smart contracts.
Ropsten test net is one of the test nets for ethereum where you can test your smart contract to see if it does what you want it to do.
You don't have to use the ropsten testnet, there's another called Kovan, and you can also deploy your own ethereum block chain locally on your computer using geth.

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@Thirdspace:
https://www.antpool.com/user/prioritiseTransaction.htm?m=savePrioritiseTx
You'll need an account on the website though.

On the console enter
If it doesn't return true then that address didn't come from your Electrum wallet.

You'll have to check if you have a malware/fake Electrum or a clipboard malware that changes any Bitcoin address to that of the hacker.

No, Electrum uses a different (and superior) derivation scheme than BIP 39 so it's not possible to generate it on another wallet, however the Electrum source code is open so it's not possible for Electrum to completely disappear from the internet.
Also, you don't need internet access to access your private keys on Electrum, so you can always export your private keys from Electrum and import them into another wallet even if Electrum servers are shut down.

To set fees manually, go to Tools, then Preferences
, Tick "edit fees manually"
Then you can set a fee you want.
Note that  0 fee transactions won't be relayed by nodes on the Bitcoin network since 0.15

You don't need to backup your wallet once you receive coins if it's a HD wallet.
Also backup of wallet is the same as backing up your p private keys.

Check the lower right hand corner of the screen, is the icon green or not?

It's not possible to retrieve it any other way.
When you set up the wallet for the first time they tell you this.
So unless you can successfully recover the json file, your funds are gone.

It depends on what exactly you want to store.
If you need access to your Bitcoins then you just need your wallet.dat file as it is what contains your private keys.

If you want to copy the blockchain itself, eg to bootstrap another computer, then you just need to copy the chainstate and blocks folder.

Technically, Bittrex can recover your Bitcoins but that would be at a security risk to them as they would have to import the private key for their usdt wallet into another Bitcoin wallet, which threatens the safety of the private key which is why they'd only do that if the amount exceeds $5,000
Only Bittrex can help you because only them have the private key to that address.
If they refuse to help you because your transaction is joy up to their limit, then there's nothing you can do.

No one would attack your coin for no reason because of opportunity cost of just using that hash power to mine (another coin) legitimately.

You could look into merged mining with another coin eg BTC.

Public key is not the address -- the address is a HASH of the public key.

As has been explained before in this forum, that's just an integer overflow check.
The number of coins isn't explicitly stated but depends on the block reward and halving schedule.

Bitcoin Core has been HD since 0.13.0