First of all, please make sure your Database server is up and running. I was getting the same error, after trying all the answers listed here I found out that my Database server was not running.
You can check the same from MySQL Workbench, or Command line using
mysql -u USERNAME -p
This sounds obvious, but many times we assume that Database server is up and running all the time, especially when we are working on our local machine, when we restart/shutdown the machine, Database server will be shutdown automatically.