There are any number of reasons. Anything from low on transmission fluid ("Toyota Genuine ATF Type WS" only!!) to blown fuse or even transmission failure (just to name a few). I assume you checked the parking brake is not engaged (trust me, we've seen everything!) About the only thing you can check in the driveway is the fluid level - DO NOT OVERFILL! The vehicle does need to be checked out to diagnose the problem, even if it's just low on fluid (to determine the source of the leak) to prevent further damage.