You can change it back using below code:
EXEC xp_instance_regwrite
N'HKEY_LOCAL_MACHINE',
N'SoftwareMicrosoftMSSQLServerMSSQLServer',
N'LoginMode',
REG_DWORD,
2; -- only difference is right here
You have changed authentication mode to windows only. So now you have to log in using windows account then you can change the authentication mode to mixed mode again. Or you can edit your registry to do so. Please check below link:
https://www.top-password.com/knowledge/sql-server-authentication-mode.html
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