5 speed transmission problem in Kia 2002 rio?

today while driving home i put it in neutral to pull up to red light. when i let go of the gearshift it just fell over.the engine didnt die, but when i tried to put it in first it sounded like the car was actually in much higher gear. i feathered the clutch to get up to speed, and got on the freeway without shifting. by the engine noise/rpm, im judging i was stuck in 4th. i drove to the mechanic shop which is closed and parked it. when i did this i tried to put it in neutral and it wouldnt come out of gear. so, somehow, my gearbox broke and im stuck in 4th?any idea how this happens? any idea how much this costs to fix? any help is appreciated. i took it to a shop ive used once before but i dont want to get burned, ya know? thanks :)