I need help diagnosing my 1998 gmc jimmy

Hello, Check the oil. There is likely coolant/water in it due to a blown head gasket. The billowing of white smoke is burning off coolant, and the engine probably died from loss of compression. The engine not turning over with the key does not make much sense if you can turn the engine with the serpentine belt, unless the crankshaft pulley is not turning. Try turning the crankshaft with a socket.