coolant problem

i own a 2000 sunfire, i noticed the other day that i had very little heat in the car, when i went to chect the coolant it was empty, my 1st thought , i blew the head gaskit, but nothing i did an oil change and found nothing, that my be the heater core, but i was told that it would produce a bad smell, what else could it be? i was also wondering , there a hose near the top of the coolant cap, it is connected to nothing, should it be connected to something?
please help
oh ya, after i filled up the coolant, the next mornning, when i went on the highway , at 120km/h it would not pick up speed, it would rev high but not kick in, than after it warmed up it was fine
Thomas