The hatch release on the Prius is actuated by an electrical button. This may have gone bad. I would confirm by opening the back hatch with the doors unlocked to make sure it is not a Smartkey issue. If it is the smart key, change the battery in the FOB. It is nearing 5 years old. That is about how long my batteries from Toyota lasted.
you can manually open it from the inside if you get under the trunk cover.