MSSQL Error 'The underlying provider failed on Open'

Question

I was using an .mdf for connecting to a database and entityClient. Now I want to change the connection string so that there will be no

Answer 1

If you happen to get this error on an ASP.NET web application, in addition to other things mentioned check the following:

1. Database User Security Permissions (which users are allowed access to your database.
2. Check your application pool in IIS and make sure it's the correct one that is allowed access to your database.