Hello Jeong Young il
If the account is a local administrator and not tied to a Microsoft account or domain, recovery becomes quite difficult. First, I recommend checking whether there are any other local administrator accounts on the server - if so, you can log in with one of them and reset the password using the net user command.
If remote management tools like PowerShell Remoting or RSAT are enabled and accessible from another machine, you may be able to connect remotely and reset the password. In some cases, preconfigured scheduled tasks or services running under SYSTEM privileges can be leveraged to execute a password reset command.
Unfortunately, if no alternative admin access is available and remote tools are disabled, Microsoft does not provide a backdoor for local accounts. In such cases, recovery would require bootable media or a clean OS installation.
Harry.