NOTHING WILL HAPPEN.
You should be far more worried about lightning hitting you or someone bumping into a window on the 30th story of a building and the glass slicing your skull in half. These are far more likely to happen, and are far more likely to ruin your day than anything the LHC is going to do.
Also, everyone is going to be posting "what a waste of money!!!" when they don't find the Higgs within 20 minutes of turning it on.
Boy, I'm sure glad that the Top quark ended the world as we know it, which these same chicken little postulated at the time.