Q: iMac shows " Enter your system lock PIN code to unlock this MAC"

Someone has locked it using Find my Mac/phone on the icloud system. You'll need that unlock pin code or take the system to an Apple store along with proof of purchase.

This just happened to me. Was it a 6-digit code? I never remember setting up a code like this. I took it to Apple's Genius Bar and was told that it was remotely locked through iCloud and they could not help me. I tried typing in 123456 for the lock pin and my iMac immediatly restarted and was then showing an "OS X Utility" screen. I tried clicking the option reboot OS X and it began reinstalling Mavericks (which was the operating system I had currently been using). After this finished, it began asking me questions to set up "my new mac". So I guess I was able to bypass the lock pin code, however everything has been erased apparently! But I guess this is better than having a computer that is locked and useless.

Has anyone else had this problem recently?

Yes!!! after googling for i just stumbled accross your post my mac just rebooted without any installation or any thing...  my mac just restarted normally "123456" just worked like a charm.  thank you very much

i did end up with this screen after a power outage, thank you so much for this mystery password

at least now i know i need to re-install the os