According to Dave Meltzer of the Wrestling Observer, Sting suffered what is believed to be a serious neck injury during his Night Of Champions Match against Seth Rollins. Still according to Meltzer, the injury occurred when Sting took the buckle bomb form Rollins.
Not much more is known for the moment, and, as you can imagine, lots of speculation as to whether this could be a career ending injury. More on this as soon as official news come out.