I was driving down a steep hill with two curves. Shortly before the first curve near the top of the hill I attempted to decelerate by pressing the brake pedal. There was no response whatsoever. I tried a couple times then realized that I had to do something different. I put the car into neutral and then actually reverse and nothing happened. I didn't hear the engine change or anything. I tried the e-parking brake to no avail. I then tried the power button for a few seconds. All the while hitting the brakes. By this time I was near the first big curve and had to steer. Given the terrain I didn't think I could drive onto a curb. After the second curve I was in the oncoming lane and noted a car coming up the hill towards me. I was able to change lanes and pump the brakes once or twice more as I got to the bottom of the hill at a high rate of speed. I plowed through the T Intersection and through a guardrail overlooking the a steep drop off to the farmland below and a ditch. Evidently I rolled the car at least once before coming to a stop and notifying emergency responders. The car is currently at the USAA lot waiting for an inspection by someone (figuring out who to engage). There was a general warning light on when I started the car shortly before the accident. I assumed it was due to condensation on cameras preventing the rear cross traffic sensors. The light went off when put into drive and I didn't have any other symptoms.