What do you mean "I wasn't able to log in"? What error message did you get?
What instance name did you use? For a default instance, you should have been able to use "localhost" or the name of the server if connecting locally (not "mssqlserver" , by the way -- unlike explicit instance names, the default "mssqlserver" name is not used for connections). If connecting remotely, you should be able to use DNS name/alias or IP address.
Were you using SSMS? Directly on the server or remotely?
Did you use Windows authentication or SQL Server Authentication? If SQL Authentication, was that enabled during install?
If you use an instance name like "12345inv", you will have to connect to ServerName\12345inv.