Solution: Solution assumes you still have access to the email address associated with your Amazon account. Go here and enter your email address. It will ask you to enter an OTP code that will be sent to your email. Check your email, get the code and enter it; you will then be given the opportunity to change your password AND if needed also the phone number (e.g. new local SIM card or other phone number that does work) associated with your Amazon account.
Case 2: You can still log in to your account but your account has been locked because Amazon thinks you may not be the actual owner of the account.
Solution: Go here and follow the instructions.