i shut off my car and could not get the key out.. i moved the car later and found out that when i parked on a small hill it would not hold the car.it made a clicking noise as it rolled backwards.
does the whold transmission have to be replaced to be fixed? and is it reliable to still drive long distance on?

 

More specifics on the vehicle always help, year, trim, etc.
However a fairly general suggestion would be to check the shift cable adjustment.
Here ya go.

 

The PARK gear or pawl is worn. NEVER trust just the park gear, ALWAYS use the parking brake too. And no, the whole tranny isn't bad but, you'll have to pay some good money to have the park gear or pawl replaced because they have to remove the tranny and crack it open.

 

By the way I've got a auto trans out of a 96 accord lx just sitting in my tool shed

 

Doubtful. Since the key could not be removed either it sounds more like the shifter is not in PARK.

ndraised scroll down til you see 'Key Stuck: Removing the key from the ignition switch or fixing seizing problem.' It's pretty much what dglassmyer posted. Peruse the page for anything else you may need to verify.