2008 Prius won't start

I have a 2008 Prius. I took it for a ride to Home Depot which was about an hour away. It was acting a little funny when I first started out. There was an occasional hesitation. It seemed to go away. But when I got to my destination and was ready to go home it started acting really weird. The engine wouldn't stay running unless I kept my foot on the gas and it wasn't charging consistently. I foolishly decided to try and make it home about an hour away all hilly back roads. I was trying to get over this big hill. I was almost to the top but the car kept stalling. I kept pushing the on button. It would go 5 or 10 feet at a time then stall. Finally it wouldn't go anywhere so I called a tow truck and had it towed to my mechanic (He doesn't know much about hybrids). He scanned it and got a code for the coolant temperature sensor. He replaced the sensor but it still won't start. The battery is at 15 percent. I bought a grid charger, I'm supposed to get it in a few days. I'm hoping the problem is that the hybrid battery is just too low and that it will start after I charge it. Does anyone have any input they can give me as to what might be wrong? Any would be appreciated. Thank you