It's been working fine so far and I did not install anything new on my PC, so it was kind of strange...
To see what is going on I looked at Event Viewer (eventvwr.exe) and found the following error messages.
Server TCP provider failed to listen on [ 'any' <ipv4> 1433]. Tcp port is already in use.
TDSSNIClient initialization failed with error 0x2740, status code 0xa. Reason: Unable to initialize the TCP/IP listener. Only one usage of each socket address (protocol/network address/port) is normally permitted.
So it looks like other application occupies SQL Server default instance port TCP 1433.
To find out what is using 1433, I ran netstat.exe as follows:
I found one process using TCP 1433 and its process ID was 5136 as you can see at the end of the output above.
And now I ran Process Explorer (procexp.exe) to check out which application it is. (Sure, you can run Task Manager or any other process monitoring utility) It turns out that PID 5136 was Google Chrome (surprisingly!) and it was connecting to Google owned 1e100.net server (126.96.36.199).
I killed Google Chrome and restarted SQL Server... and everything worked fine...