Toyota 2005 Prius Owner's Manual page 118

If the indicator or buzzer comes on...
(g)
(h)
(i)
(j)
(k)
(l)
Key reminder buzzer
P position reminder
(m)
buzzer
If any warning comes on, the hybrid system may not start by pushing the "POWER" switch. In that case, push it once again.
118
Take vehicle to Toyota dealer immediately.
Close all doors.
Fill up tank.
Replace engine oil
Take vehicle to Toyota dealer. If red brake system warning light is also on, stop imme-
diately and contact Toyota dealer.
Remove key.
Push the "P" position switch.
Do this.