Do you only get this error after 6:30 pm? Are you a member of sysadmin on the server? If you are, there should be no reason you can't get on. If you're not, speak to the administrator of the server. Alternatively, it may be something that doesn't have anything to do with SQL Server, such as a problem (or planned outage) on the network. Can you even ping the server when you get that message?