El Capitan Help! I'm stuck on a screen with a circle slash!!!

I have a MacBook Pro (2011) and I recently gotten my computer fixed with a new hard drive and what not. I went to install El Capitan only then to see that as soon as it finished downloading it, it just freezes and flips to a white screen with a circle slash through it. I haven't installed Yosemite because I wasn't so keen on installing such a bug infested program. so when I got my laptop fixed I knew I should install El Capitan because on what my father has told me. So now it's almost 6 hours later and I get this. I am so afraid on what has happened to my computer, I did back up all of my important things to flash drives and to an external hard drive. But now I don't know what to do! I've restarted it and the white screen loading line is like at 90% done but it's not moving. After a few seconds the screen flips to the white screen with a circle slash. Please help me. I need to know what's wrong with my computer before I go back to college in a few weeks!!!!