The small 12V battery is dead. Most likely you didn't shut the hatch completely and the light in the cargo area drained it. The best way to prevent this happening is to always lock the Prius.
You can easily jump start it from the jumper terminals under the hood but you'll need to replace the battery ASAP because once a lead-acid battery drains completely it never holds as much, and anyway about five years is all you can expect from a lead-acid automotive battery.
Some Prius owners put in an Optima Yellow Top battery. It's reputed to take 300 full discharges before it dies.
http://www.elearnaid.com/12vo1topraub.html