Hi,
I'm sorry to hear that you're experiencing this issue.
If your account has been hacked, 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 it doesn’t 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.