99 Galant problem

HI,
I was driving my 99 Galant on the highway at 75mp and suddenly the speed going down .. then i stopped and shut off the engine. started again and put in drive and gave gas , the car does not move but the flywheel is rotating ( I mean nothing wrong with engne..) what is problem??

Thanks

could it be the driveshaft? or perhaps a tranny prob? I'm not car expert, just kinda thinking aloud! Good luck, that's gotta blow!