This happened to me sometime back.

I want to try out a WordPress plugin in my local machine before installing on a production server. It is an Ubuntu machine. Downloaded and installed Xampp, then setup WordPress with MySQL. Now tried uploading the plugin zip file, it throws some permission error, asking to fix permissions or use FTP. I thought of just chmod 777 recursively for the WordPress directory to fix this easily.

Ran the command, looks like it is hung. Terminated using Ctrl+C and then ran the same command. Again it is taking much time. It should not take so much time to recursively change the permission of just a WordPress directory. Thought something was wrong. Before I realized the damage is already done.

Looks like I ran the command

sudo chmod -R 777 /

instead of

sudo chmod -R 777 ./

Fuck, I missed a dot in the command and it is changing permissions of everything in my machine. Saw the System monitor, CPU usage spiked to 100%. I can't close or open any program. Force shutdown the machine using the power key. It didn't boot again. Recovery mode didn't help. Looks like there is no easy way to restore back from this damage. Most of the files I need are backed up in the cloud, still, need a few more personal files so that I can format and reinstall Ubuntu. Realised I have Windows in dual booting. Boot into Windows and used some ext4 reader to recover the files, formatted and reinstalled the OS. Took a few hours to get back to my previous setup.

Lesson Learned: Don't use sudo unnecessarily.
Double check the command while executing.
Running a wrong command with root permission can fuckup your entire machine.