There are only a handful of codes that will prevent you from starting your car. The most common is a P0AA6 - HV Leak (assuming you haven’t run out of gas!). This means that some part of the insulation between the high voltage system is "leaking" voltage to car ground (chassis). It is typically in:
Unfortunately, unless you have a code reader on-site, getting codes read on a Prius that won't move is impossible. Your best solution is to disconnect the 12V battery to clear the codes as follows:
Remove the mat
Remove the cargo tray cover
Remove black plastic cargo tray
Remove cover over 12V battery in the passenger rear corner
Remove purple (maroon?) plastic covering battery terminal.
Disconnect the center of the three connectors.
Leave it disconnected for 2 minutes - don't close the hatch!
Replace black tray, cover and mat, but leave the covers that had been on the 12V off for access - this way you can just open the hatch and disconnect the 12V if needed.
This first minute of this video shows steps 1-4 above.
For step 5:
For step 6, user your finger to hook the underside of it, remove this plug:
After connecting the 12V, you may attempt to get the car in ready mode; however, it's a little finicky after the 12V has been disconnected. Do it as you usually would, but you may need to press it 2-3 times about a second apart.
This will allow you to start and drive the car 99% of the time. If you don't succeed on the first try, and the warning light returns immediately, please try 2-3 more times ensuring you are leaving it DISconnected for at least 2 minutes, as it really is 99% successful. The #1 mistake is not leaving the 12V disconnected long enough. The #2 mistake is not pressing the ON button 2-3 times, 1 time/second until the car goes ready.
Once you are able to get the car started, drive it normally until the light comes on. Once the light comes on, drive STRAIGHT to O'Reilly. DO NOT TURN THE CAR OFF. Keep the car in ready mode and ask them to check the codes for you. Get the CODE(S). Don't let them tell you the description. I recommend O'Reilly because they use Bosch code readers that are usually very successful in reading codes that others won't.
If it's a P0AA6, you can continue to drive the car that way by disconnecting the 12V and reconnecting. It's a hassle, but it's the difference between being inoperative and mobile. Toyota disabled the car for a reason, so there is a legitimate safety concern, but I would personally never choose being stranded over being able to return home or to the shop.
A recent customer drove his car for several weeks before correctly diagnosing the code and coming to see me. He used a hand-held OBDII reader to clear codes rather than resetting the 12V above.
It is important to diagnose the issue in short order. As noted above P0AA6 can be triggered by many causes. Typically only the dealer (or Phoenix Hybrid Batteries) can read the sub-codes that point to the different potential causes.
The dealer typically charges more than their normal fee for this code due to the far more involved diagnostic process. I can either exclude or confirm that it's in the battery in about 5 minutes.