At times this will happen when you change ports for mysql server or even without no change and then you resort restarting your machine and that helps.
Most likely an orphan myqld process is running in the background that does not allow any new one to start. You will have to stop(kill) that runaway process and then startup using the control panel.
1. Launch Activity Monitor tool on your Mac.
2. Search for a process called mysqld from the list.
3. Click on that process and press the Quit Process button.
Now restart mysql server via the control panel.
You can also kill the process using command line as
>sudo killall -9 mysqld