Hi! is the problem that the key will not turn? or it turns and nothing happens?
My wife has a 2004 G20 that had the same exact problem.
The key cylinder is damaged. Unfortunately it must be replaced, and a new key cut and programmed. Cheapest way to attempt the problem is call a local locksmith to attempt to free the tumblers in the cylinder, then you are on your way. If came apart, a new key cylinder (I paid 180 on ebay for a new one) and a new key must be cut at the dealer and programmed to your anti-theft module. (No way around it, I tried, promise you.) Which may result in a tow. Now, if you can get it started, do not turn the key to the "off" position to avoid a tow. A way to free it up is wiggle the key like crazy while putting it in the cylinder and pulling it out. Helps to move the steering wheel a little too.
To get in gear to push if needed, there is a little tab to the right of the shifter that can be popped up with a screwdriver or small pin, and push a small screwdriver into the hole to release the shift lock.
I hate to be the bearer of bad news, but this is the only way out. (Trust me I know).
Although this answer may not be the one you like it is correct and an accept is extremely appreciated.
Thanks so much,