Login failed for user

If I log in from application asp.net to to server ms sql server appear this message

Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection

I bet you use Windows Authenation in your connection string. This error indicates that SQL didn't 'recognize' the connectied account from asp.net application. For example, if you deploy your website under IIS and connect to SQL server, actually IIS will use 'ASPNET' account (or 'NT AUTHORITY\NETWORK SERVICE' for IIS6.0) to connect to SQL. To solve this problem you need to add the account to you SQL logins. Please take a look at this article:

http://msdn2.microsoft.com/en-us/library/134ec8tc.aspx