When im shifting gears with my car (2005 Pontiac Grand Prix), when its in reverse or drive, it wont move. Why does that happen?

You need a new trasmission...go to car-part.com you can fing a transmission for as low as $400. However before you call them some around you area for local mechanics who can install it. I have a guy in my area who can put it in for $300. Then after you find the right guy to do the job call the place to have the transmission delivered to the shop of you choice. That is how you get a transmission and installed for under a grand. Shhh...dealer secret...lol.

when i rebuilt mine, it cost about 1800. it sounds like your first gear dogs got sheared off. the tranny in these cars is alot like an indirect drive, where there are splined gears which slide back and forth, freewheeling gears which the splined ones activate, and machined ones, which are solid pieces of the shafts themselves. when the dogs shear off of the machined gears and the splined gears, the freewheelers have nothing to catch and the car won't move. the labor for this job is usually around 1000, but if you're savvy and shop around you can find a good transmission specialist and they'll find the best deal for you on parts (just don't go to a brand shop- tuffy's firestone, midas, etc.) and even let you customize your ratios if they're that good. (my guy was, 1800 for a custom racing 4 speed with a double over, making it an 8 speed)

check the fluid level first, if its low it will slip very badly

Ive got a 3.8 L v6 engine. If anyone was wondering. I work at a dealership so im having one of the techs take a look to see if it is the tranny itself or just the fluid. And an fyi it doesnt always do this. It happens every once in a while.

Kylie - I have a 2006 3.8L v6 that is doing the very same thing. Did you ever find out what the actual problem was? I see a lot of conjecture above but no definitive answer.