Help!

Don't know if this pertains, but I had the same problem on an HHR which was compounded by the ignition not unlocking the keys. There was a switch in the shifter housing that didn't get flipped. Had to remove the trim around the shifter, cut some of the plastic frame to the unit, and then flip the switch back into place with the stick in neutral. Happened 2 or 3 times while I had the car. If I had taken it to a mechanic, I would have been told it was a nonservicable unit, and paid several hundred dollars to replace the shifter unit.

So something similar likely broke in your car. It could be a simple fix or a PITA depending on the exact cause and ease of getting to. If the the car drives fine, and you can turn it off and remove the keys/fob, I personally would only bother trying to fix it if required for inspection. Assuming the control is entirely electronic like a gen2, you don't have to worry about the gates on the shifter not working. Accidentally shifting from drive to reverse will result in going to neutral instead of a chewed up transmission.

Just get in the habit of using the parking brake. It is pretty much all manual transmission cars have to keep them from rolling around. I started the habit with the gen2 Prius because the Park position had a larger range of give than previous cars, and now I do it with all my cars.

There is also a gen1 subforum you can check for info if you haven't already done so.