Lock editing on node except for Admin
It sounds like you could accomplish this with a combination of rules and user roles. I assume to don't want the node to be editable by anonymous users so set the rule to grant editing permissions to all users who have a particular role when the value of the field changes from locked to unlocked (or whatever your corresponding value is). If you want more granular control, (i.e. when a node is unlocked some fields are editable but others remain editable only by admin users), you'll need to install the field permissions module (https://drupal.org/project/field_permissions.