2000 Lesabre Emergency brake lock up

It couldn't have possibly engaged by itself, unless something has happened in the rear park brake assembly. When you engage via the lever, is there a noticeable amount of travel with little, or no resistance felt. Is it possible that it was applied after driving, then the temperature dipped below freezing, causing the moisture to freeze, and freeze up the cables? If this is the case, you will need to park it in an area that is above freezing to thaw, and it should release.