Car randomly won't start (2005 Toyota Prius)

Hi! My mother has a 2005 Toyota Prius with approximately 143k miles. It
runs and drives amazingly, and the hybrid battery charges as it should.
However, every once in a while, it will not start and will display the "red
triangle of death". The car will still turn on electrically, but will not start the
engine. We have only resolved this by disconnecting the 12 volt car battery
for about 10 seconds, giving the car a soft reset. All in all, the only code it
has been shown to throw is P0442 (small evap leak). Does anyone know
how to resolve this?