All of a sudden CRM stopped working, deployment manager started to give this error and when i tried restarting the CRM async services it failed. I somehow managed to fix this issue by adding port 1434 in Windows Firewall Security on database server. Please follow my previous blog which contains the steps to add port in firewall inbound rules.

https://community.dynamics.com/crm/b/innovativeaj/archive/2018/03/28/a-network-related-or-instance-specific-error-occurred-while-establishing-a-connection-to-sql-server

Now check your CRM, it will be working perfectly! :)