Prius won't start and check engine light appears

Small point of order that may help clarify it for folks. IG-ON
displays the battery SOC even when the main HV relays are not
connected. Why? Because the battery ECU that's inside the battery
box can read it. It has all these little sensing taps into the
battery string and its own memory of the state of charge, so when the
main 12V power and networks come online in IG-ON it can communicate
with the other computers and the MFD and display the battery info.
.
IG-ON is useful for just that -- powering up all the computers
*without* necessarily trying to bring the HV battery online or fire
up the whole running system. In that mode you can usually hook up
a diagnostic tool and get codes, clear errors, etc without burning
fuel. With the caveat, of course, that you're running all that
entirely off the 12V battery and nothing is charging it, so don't
stay in that state for too long.
.
You can also get into Neutral in IG-ON mode [but no other gear]
and push the car around.
.
_H*

 

Same thing happened to my 2005 after cleaning out the car and leaving the radio on too long. Wouldn't start and check engine light went on. Nothing worked till I tried the depress and hold On button for 10 seconds "trick" suggested above. Car started although I don't quite understand the mechanism at work here. Perhaps pressing an holding "On" is a type of reboot? IN any event, it worked.

 

Pressing and holding down the power button is like the "forced reset" found in laptop computers.