Unable to connect to configuration database

The main reason behind this exception is due the SharePoint server  inability to contact server. This may be due to

• SQL server/Cluster is down,
• Network is down.
• SPN not configured properly

If all above are correct and SQL server box up and reachable using ping command from SharePoint servers, the go to command prompt and type the following command

C:\> IPCONFIG /flushdns

Type Services.msc in Start Menu--> Run and hit enter to open services mmc console.

Restart services named netlogon, dhcp client, and dns client.


Also if your server are VM boxes, then make sure system time in all servers are in sync with the server hosting active directory.