Engine stopped and vehicle went into neutral. Family of 5 in the vehicle on a busy highway. Once pulled over safely from the highway and shut off ignition, the vehicle stayed in park and would not shift to neutral, the engine would not restart. Vehicle had to be towed into the GMC dealer. No warnings, messages or symptoms prior to the failure. The vehicle was just at the dealer for routine maintenance 4 days prior. Dealership found that the engine had seized.