Microsoft SQL Server Error: 18456 occur

Hi all,
When I try to connect to the SQL server 2008 R2, I ended up with an error message. The error says it cannot connect to local host and it says the login failed. However, I have the authority to access the server. In that case, why I got this error? Please help me to resolve this error and to find solutions. Thanks a lot.

Connect to Server
Cannot connect to localhost.
Additional information:
Login failed for user ‘ADTESTres’. (Microsoft SQL Server Error: 18456)
