I've downloaded OS X Mountain Lion and now my mac won't even turn on

It restarted by itself and then wouldn't turn on so I tried manually turning it on by myself and it just showed my desktop wallpaper for about an hour. I tried turning it on and off again but now it comes up with all this writing then a window pops up and says "you need to restart your computer. Hold down the power button until it turns off then press the power button again to turn it on" and every time I do so, the same thing happens again where all the writing comes up across my screen then that window comes up. I've tried holding down command + r and also tried holding down the "option" key. What do I do?