Is the key turned to the right position? Make sure the slot is straight up and down, if it's sideways, it's in valet mode, and you can push all you want, and it won't open.
Other than that, I hate to say it, but there is a plastic grommet that holds some linkage for the lock, and it comes loose over time. You'll have to drill a hole behind the license plate to manually unlock it. Don't drill unless you know where to do it at. You'll basically have to find another car, take it apart, and look for the location that you'll need to be able to stick a rod in and unlock it.