Error 18456 is the generic error message for Login Failures in SQL Server. Depending on your situation this could be a simple misspelled password or the windows account does not have permission to the Instance and/or database. However I am not looking at those simple issues around this SQL Server error message.
“Error: 18456, Severity 14, State 38. Login failed for user 'DomainServiceAccountName'. Reason : failed to open the explicitly specified database.