How to recover from a Frozen MAC ?

Mac OS is usually quite stable and not prone to a system crash, yet seldom it does get stuck/frozen. Lets check out what can you do to recover from your frozen MAC without having to electrically reboot.

Force Quit an application

• In the Dock, right click the offending application and select quit.
• If that doesn’t work, Ctrl + click the offending App, and then Hold Option
  That should turn the ‘Quit’ in the menu change to ‘Force Quit’.
  Select ‘Force Quit’ and BAM! The app should quit immediately
• Alternatively, you can use the famous Window’s Ctrl + Alt + Delete version for MacOS viz pressing Command + Option + Esc simultaneously to open the Force Quit Applications window as shown below
  

You could force-quit the desired application there-in. You may see (Not Responding) next to the offending application.
99% of the times, this should get your MAC back to normal.

An app that refuses to force Quit

If the unresponsive app declines to close, your only option is to restart your MAC. You can restart the usual way via the menu.

• Unresponsive Menu

Use Cmd + Ctrl + Eject to restart your MAC if your menu goes unresponsive too.

• Last Resort

If both of the above fail, press and hold the power button for a few seconds to force turn off your mac.

Activity Monitor

Too many applications/processes running simultaneously in the background may cause the system to lock up causing a frozen Mac.
The activity monitor is the equivalent of the task manager in windows. Launch Activity Monitor via spotlight search(Ctrl + space and type Activity Monitor) to see the list of running applications and processes and see detailed info about the CPU, memory, disk, network,.. usage.

You can close/force-close unrequired applications by clicking the cross on the top left of the window to free up memory. To make things permanent, remove unnecessary applications from automatically starting up at boot.

Frequently Frozen MAC

If your OS freezes frequently, it could (but not necessarily) point to more grave problems. You can troubleshoot in the following order to rule out possible causes:

• Shortlist a particular App that causes Freezes. Try to update the app, or reinstall it. Or disable any 3rd Party plugins for the App.
• Boot in Safe mode by holding Shift during boot up and see if the freezes go away. If yes, you can rest assured that your MAC doesn’t suffer from a hardware issue.
•  External Devices (or device drivers). To begin, you could try disconnecting external displays/printers/etc and see if the freezes disappear
• Low Hard Drive Space. Ensure you have sufficient space on your hard-drive. I recommend to have free space of about 30% of your total capacity.
• You can run Repair Disk utility in Disk Utilities to clean up software problems with your HDD.
• Restart your Mac holding the Cmd+Option+P+R keys. This clears out the NVRAM/PRAM, which usually stores persistent settings. This would restore default values and may resolve the issue. You should also reset the SMC (another place where system settings are stored)
• To check for hardware issues, you can run Apple’s official Hardware Test (AHT) to get to the source of the issue.

Prevention being better than cure, an interesting article summarizing how you can prevent your mac from freezing.

Reference