I'm sorry to hear that you're experiencing this issue. I'm happy to help.
The hacker changed your username and possibly your email address, which is why your account now appears to no longer exist.
Yes, this can happen if someone gains full access to your account. They can update the account’s security information, including the username, password, and recovery details, effectively locking you out.
If your account has been hacked, please use the sign-in helper tool in the link below to guide you to the appropriate solutions. The link provides comprehensive steps to help you recover a hacked account and also gives you the option to chat with Microsoft Support as a last resort.
https://go.microsoft.com/fwlink/?linkid=2196140
Also, please fill out the account recovery form: https://account.live.com/acsr
Microsoft takes account security seriously, so the recovery process is fully automated to prevent unauthorized access. Complete the form using a device and location you’ve used before and provide an accessible email address.
Microsoft usually reviews requests within 24 hours. If approved, you’ll receive instructions to recover your account.
You can submit the form up to twice within a 24-hour period and add any new details you remember.
If none of the steps work, I’m sorry to say that since the username has already been changed to a different one, it means the account has been taken over by the hacker. There is no way to recover it, and it is now unfortunately lost.