Yesterday we received a call from one of our clients, please help, during the night our SharePoint farm stopped working, we can not even start Central Administration.

We went to the clients site and checked what was going on in event logs. WFE was saying that when it was trying to connect to the database, access was denied?? Nobody changed settings and access denied was even for Config database. We logged in to SQL Server, but no errors there. We did notice one thing though, Server was restarted during the night due to Microsoft Updates.

So what we did was, we ran the Products and Technologies Configuration Wizard, which finished successfully, and everything started working again.

Apparently, when SQL Server was restarted SharePoint lost connection, which couldn't be established even after server restart and only Products and Technologies Configuration Wizard helped to get the farm up and running.