Account Hacked

You did exactly the right thing. Now, see:

What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

And see these security-related Apple documents:

 If you think your Apple ID has been compromised https://support.apple.com/en-us/HT201303 and Security and your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

Using app-specific passwords https://support.apple.com/en-us/HT204397

ShAd0W1982 Said:

"Account Hacked: Hi recently got an email to say someone signed into my account using a Mac, I only own a Apple TV4k and have never used a Mac in my life. I'm a bit worried about this happening as it seems someone had hacked my account?"

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Modifying your Credentials:

With Apple Account security being of high-concern here, I'd consider modifying the following:

• Modifying: your login credentials (Using Both a New eMail and a New Password for each and all sites)

• Associating: a different method of payment to your account

• Enabling: Two-Factor Authentication for Apple ID and for all other accounts that have such an applicable option

Turn on two-factor authentication. Your messages with codes nobody never don't read