The 12V battery is likely drained. Use the jump start terminals under the hood to jump the battery--be sure you get the polarity right.
Before jumping you should also check the cargo area to make sure the 12V battery is actually connected. Some people disconnect it for long term storage.
When the Prius is powered down, the traction battery (the large battery) is totally disconnected. It takes a very long time for it to drain to the point where it won't start the car. Six months appears to be no problem if the battery was at the normal six bar level when the Prius was powered down.
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