I turned on my computer and all I get is a grey screen with the apple logo and the message "Enter your system lock PIN code to unlock this iMac". I never created a PIN code...I have no idea what and why it is all of a sudden requesting this information. Any advise is greatly appreciated.
The machine has been remotely locked by its owner, or by someone in control of the owner's Apple ID account. If you're the rightful owner, take it to an Apple Store to be unlocked. You may be asked for proof of ownership.