Need some help

i think i may know what the problem is, but i want to get some input from you guys:

my wife drives a 99 cavalier and yesterday while driving she says that suddenly the temp guage plummets and the car starts smoking and the check engine light comes on...i check it last night and it looks as if all the radiator fluid went out of the overflow...i'm thinking it's the thermostat but i'm not sure if that would cause the temp to fall and not raise...so, any help

Get the check eninge light change. change the thermy and jack up the side of the car while you are adding fluid to try and get the air bubbles out