Cannot open database "DBName" requested by the login. The login failed.
Login failed for user 'DBUser'.

This sometimes happen when you attach a database from a different machine
(because the security doesn't reside inside the database itself)

Solution:

use <<DBNAME>>
go

---create LOGIN <<USERNAME>> WITH PASSWORD = '<<PASSWORD>>' , check_policy = off;
---OR (uncomment which one is needed)
alter LOGIN <<USERNAME>> WITH PASSWORD = '<<PASSWORD>>' , check_policy = off;
go

exec sp_change_users_login 'Auto_Fix', '<<USERNAME>>', null, '<<PASSWORD>>'
go

Make sure your username and password match the web.config file's: <configuration>
     <connectionStrings>
          <clear/>
          <add name="MyConnectionString" connectionString="Data Source=MYPC\SQL2005;Initial Catalog=DBNAME;Persist Security Info=True;User ID=<<USERNAME>>;Password=<<PASSWORD>>" providerName="System.Data.SqlClient"/>