SharePoint error cannot connect to the configuration database

When working with SharePoint, you may come across an error "cannot connect to the configuration database" while browsing your site.

This error may occur for a specific site, multiple sites or even sometimes for all the SharePoint sites including Central Admin.

The error message gives you few important keywords - CONNECTION, CONFIGURATION and DATABASE . So it seems, there is either something wrong with the SQL Database connection or the IIS Configuration.

Here are some possible causes and will help in resolving the issue -

1. Check the IIS Site - is it stopped?
2. Check the Application Pool - it is stopped?
3. Check the Application Pool Identity account - is it correct and password has not expired?
4. Check the Application Pool Identity account - is it missing permissions on SQL databases?
5. Check the SQL Instance - is any configuration changed recently?
6. Check the SQL Instance - the server is up and required SQL services are not stopped?
7. Check the SQL Database - the database is not down or offline?
8. Check the Firewall settings (less likely) - nothing is blocked?