Seized engine???

Yesterday, I asked about noisy air conditioner and defroster vents and someone here suggested it was probably the a/c compressor belt. All well and good, I booked the car in to have it looked at. Today, my husband is driving when he hears a loud "flap flap flap" sound and the car died and would not restart! Had it towed to a garage who say the engine cannot even be turned manually and appears to be seized!! Any ideas, help!!!???