Issue/Question
My Mac has a lock screen on it, stating:
Enter your system lock PIN code to unlock this MAC.
Cause
When a Mac is linked to a person's iTunes/iCloud account and enables the Find My Mac feature, that person can remotely lock the computer.
Resolution
The person with the linked account will need to provide the PIN to unlock the computer. If the PIN cannot be remembered, the person with the linked account will need to contact Apple's Support.