Most likely because your account was hacked. You'll need to change your password so that the hacker who guessed or found your current password can no longer access it.
When an account is blocked the main password as well as any App passwords you were using stop working. Access to your data is also revoked for all sites, programs, and mobile apps. This way both you and the hacker can no longer access your account via Yandex websites or various apps.
Yandex automatically checks your account activity. Is a message sent from your inbox spam? Was there a suspicious authorization, etc.? If you clearly couldn't have performed such actions yourself (for example, you logged in to your account from both Vladivostok and Moscow within an hour), then your account will be automatically blocked. You can only unblock it by changing your password.
For more on how hackers could have figured out your password and gotten access to your account, read Protecting your account.
If your account was hacked, then your current password is most likely known to the hacker. Therefore, you'll have to come up with a new password that only you are privy to. You should stop using the old password to prevent the hacker from logging in to your account.
What you need to do:
Before you change your password, check all your computers and other devices that you logged in to using your old password for virus. Hackers may use a virus to get your new password and log in to your account once again.
Log in to Yandex using your current password. The Change Password page will open.
Confirm that you are the account owner:
If you've linked your phone number to your account, request that a confirmation code be sent via SMS, enter it on the page, and click Confirm. If you can't access the linked number, then click on I can't access that number.
If no number was linked to the account, then answer the security question. Try to answer correctly, but if you can't remember exactly what you entered, then our recommendations can help.
You'll need to confirm your phone number after answering the security question. This will help you to quickly and easily restore access to your account in the future.
If you didn't set up a security question or link your phone number, then you won't be able to change your password. In this case, contact Support to restore access to your account.