Drilling in concrete and hitting something hard - what are my options?
You may be hitting rebar, the reinforcing steel embedded in concrete.  You can buy a rebar cutter the same size as your concrete drill bit to get through the rebar then switch back to your concrete bit once through.  However a great deal of caution is in order.  

It's pretty hard to determine whether you're hitting a very hard bit of aggregate or steel / metal, and if it is metal, it's hard determine whether it's safe to drill that metal.  In some cases, for example concrete beams, cutting the steel in the concrete can weaken the beam.  It's also possible you're hitting a water pipe, waste pipe, or an electrical conduit.