This post is actualy dedicated to connection strings and the whole using of SQL Server... But still... There are several cases in such situation: First. Your SQL Server is configured for windows authentication string only (my local choice). This means that the authentication is provided by Windows user system. An example of connection string for such situation: Server=(local);Database=<Database name>;Integrated Security=SSPI;  Be sure to add the ASPNET user in two places in Enterprise Manager: 1. in the Security node of the local SQL Server\Logins. 2. in the Databases\<Database name>\Users. Be sure that ASPNET user have the dbowner permission. The second choice is for SQL Server authentication. In this case you should specify the User ID and Password in the connection string an example:
Server=(local); Database=<Database name>;User ID=<User name>; Password=<User password>;
This string shoud appear in the web.config file in two places. The connectionStrings and appSettings. Be sure that no other connectionString specified.

  DkAmirit

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Best regards, Evgeny