after update domain controller server 2016 to 2025 can not sign in user domain and local admin

Dear Surachai Chayduangrudeeporn,

Thank you for contacting us regarding the sign-in issue following your upgrade from Windows Server 2016 to Windows Server 2025.

We understand that after the upgrade, both domain users and the local administrator are unable to sign in. This behavior may be caused by Active Directory services not starting correctly, misconfigured domain trust relationships, or corrupted system components during the upgrade process.

To begin troubleshooting, please verify that the Active Directory Domain Services role is running and that the server is properly connected to the domain. You can check this by logging in via Safe Mode with Networking and reviewing the event logs under Directory Services and System.

If the local admin account is also blocked, we recommend using recovery media to access the system and confirm that critical services are operational. Additionally, ensure that SYSVOL and NETLOGON shares are present and accessible, as their absence can prevent domain authentication.

Please let me know if any of these steps help resolve the issue. If you find this answer helpful, feel free to hit "Accept Answer" so others can benefit too 😊. T&B, Harry.