Unauthorized account access

If you can, Check your Apple ID device list (https://support.apple.com/en-us/HT205064) and remove any device that shouldn't be signed in using the steps in the link.

If there are none that shouldn’t be signed in, then you should not have a problem at this point. But, if an unauthorized device is signed in, Change your Apple ID password https://support.apple.com/en-us/HT201355 

Here is 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 to deal with cases of unauthorized sign ins and other security matters:

 If you think your Apple ID has been compromised https://support.apple.com/en-us/HT201303From the link, among other things, Change your Apple ID password and choose a strong password.

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

For Apple Support help with your problem, use:

Get a call from Apple support to help solve your problem  https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter https://twitter.com/AppleSupport

Apple ID account security for country-specific and region-specific

help https://support.apple.com/en-us/HT204169

If your Apple ID is locked or disabled https://support.apple.com/en-us/HT204106 "If you or someone else enters your password, security questions, or other account information incorrectly too many times, your Apple ID automatically locks to protect your security and you can't sign in to any Apple services.

If you use two-factor authentication, you need a trusted device or trusted phone number to reset your Apple ID password and unlock your account. If you have a recovery key, you can also use it to help you regain access to your account. …

If you use security questions with your Apple ID, or if you don't have access to a trusted device or phone number, go to iforgot.apple.com. Then you can unlock your account with your existing password or reset your password. After multiple unsuccessful attempts to unlock your account, your Apple ID will remain locked and you can try again the next day.” Eventually you will be able to enter account recovery.

So, see Recover your Apple ID when you can’t reset your password https://support.apple.com/en-us/HT204921. Read the whole document carefully.