I had that problem happen with an AX-5 right after I had put it in the truck. (used to be an AX-4, I added the 5th gear, so everything hadn't quite been lubed up yet)
The shifter for 5th/rev somehow got stuck in 5th. I was able to put it in 3rd, move the shifter over halfway towards 5th, then pull it back and get both sliders back into neutral. This is something you could not possibly think to do unless you've had one of these transmissions apart.
Take the shifter off, and take a look inside. There will be an oddly shaped piece that the shifter sat on, and that has an arm that goes down to the three sliders. Get a flashlight, and look down at the sliders, see if they're all lined up.
It's pretty hard to get a transmission stuck in two gears at once. They have a complicated internal spring system that's supposed to prevent it, but it's possible.