I had to deal with a computer problem this morning. My mum's iMac had restarted to install a security update, but wasn't coming back up to the desktop.
I got it into the Recovery mode and checked the hard disk for errors, but that reported no issues. It partially restarted and got as far as 'your computer was shut down, do you want to reopen applications?', but then was stuck at a black screen.
I rebooted back into Recovery mode and restored it from the most recent complete Time Machine backup from last night. That did the trick, and after reauthorizing iCloud and waiting for Apple Mail to import all of Mum's email, it was back up and running.
At some point, I need to look into getting that iMac running from an external SSD to speed it up — I can't upgrade the memory on it as it's the 21-inch model, but I can bypass the internal hard disk, which is the other major bottleneck affecting the speed. I'll also need to look at cleaning things up a bit, and maybe getting her back onto Safari instead of Chrome.
The moral of this story? External hard disks are cheap, and all modern operating systems come with automated backup systems built-in, so it's not hard to make sure your computer's data is regularly backed up. It has saved my bacon many times in the past, and I'm glad I made sure that Mum's iMac got backed up too.