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

This is common issue only. Even I have faced this issue. On the development machine, configured with Windows authentication, it is worked perfectly:

Once hosted in IIS with the same configuration, I got this error:

The underlying provider failed on Open

It was solved changing connectionString in the configuration file:

Other common mistakes could be:

1. Database service could be stopped
2. Data Source attributes pointing to a local database with Windows authentication and hosted in IIS
3. Username and password could be wrong.