Microsoft SQL Server Error: 18456(Administrator is also not able to access)
During the installation there is option to choose what are users to be
  added as instance administrators. 
  
  Your customer should add you or your team Distribution list. You
  probably added the built-in Administrators group and now you're
  running as admin, that's the reason you cannot access the instance. 
  
  If yes, then is simply a mater of running the client process elevated
  ('Run as administrator').


Check it out: Connect to SQL Server When System Administrators Are Locked Out

Otherwise;

try to run SSMS (SQL Server Management Studio) on the server with 'Run As Admin' to check if you added builtin\administrators during install. If you cant find any user with enough rights then you have to add a login to the server and give it sysadmin rights.

Start the server in single user mode by starting cmd.exe on the server with administrative rights and running net stop mssqlserver and then net start mssqlserver /m
then in the same window run SQLCMD. 

Use the following:

CREATE LOGIN [DOMAIN\ADMINISTRATOR] FROM WINDOWS;
EXEC SP_ADDSRVROLEMEMBER 'DOMAIN\ADMINISTRATOR', 'SYSADMIN';
GO
EXIT


and then run net stop mssqlserver and net start mssqlserver.

If you are running a named instance the servicename is different - 
Check services.msc to see the name and you have to run SQLCMD with -S server\instance -E to connect