Sonya, you have two batteries in your car. There is the normal 12 volt battery like in all non - hybrid cars. There is also the high voltage battery (HVB) used for electric operation. The charge you are looking at on the dash battery symbol is the HVB charge. The 12 V battery is required to start the car so that the HBV can be connected to the HV electronics.
What happens when you push the start button or turn the key is that the electronic modules (which all use the 12 Volt battery for their power) are activated and allow the car to start. If the 12 V battery is failing, the electronic modules may not have enough voltage to operate and nothing happens.
Do you have a volt meter to measure your battery voltage? if you do, you can open the hood and look for the positive post (has red cap on it) and a negative uncoverd metal post sticking up near the positive post which are both located to the driver's side about half way back. Measure the voltage when it won't start. It's likely too low for the car to start. The question though: is it a failing battery (very likely) or is something, like a module not turning off, draining the 12V battery when the car is off (unlikely).
Edited by Plus 3 Golfer, 07 October 2017 - 04:41 PM.