If you can still sign in to your Microsoft account, you should change your password immediately, so whoever stole your old password can no longer use it. Under Account security, next to Password, tap or click Change, and then follow the instructions.
If you can't sign in to your account, a hacker might have changed your password and taken over your account. Tap or click Can't access your account? or go directly to the Reset your password webpage, and then follow the instructions.