How do you know the engine is not seized? Have you put a breaker bar and socket on the crankshaft and turned it?
If the engine is not seized, check the battery for state of charge and the ability to handle a load. Then, check battery cable connections to make sure they are secure and free of corrosion. Ensure the transmission shift lever is in park, and use a diagnostic tool to verify the processor sees it in park. Ensure the keys used are properly programmed to the vehicle.
RepairPal Certified shops can diagnose and repair your 2002 Mercury Sable. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.