For me the issue was TCP/IP protocols were disabled (the second option in the accepted answer). However, why I chose to write my own version of the truth is because I had a tough time getting to the crux of the matter. I am on Windows 10. In short, just follow this:
a) Go To start Menu
b) Type and select 'SQl Server <version> Configuration manager'
c) Select 'Protocols for MSSQLSERVER' under SQL Server Network Configuration. Refer below image:
d) Right Click on TCP/IP and select Enable.
e) Restart SQL server services from the same window .
f) Now you are good to go.