Hello,
If your account's security information has been changed by someone else and you can no longer access the Security basics page, your account may have been compromised.
First, please refer to the following Microsoft Support article, which explains how to complete the Microsoft account recovery form, to check the appropriate recovery options:
https://support.microsoft.com/en-gb/accounts-billing/manage/microsoft-account-sign-in-helper
If two-step verification is enabled and you cannot access any of your registered verification methods, you may not be able to recover your account using the recovery form.
Please do not share any passwords, verification codes, or personally identifiable information in this thread.