When you say rock hard, what do you mean? Does this mean when you open one, it goes "Pssssssst"? Ok I re-read your post...well carbonated at 24 hours sounds scary. Test one in a day or two and see if it gushes out. If it does, you'll need to gently pry up (without crinking the cap) and release the excessive carbonation. In the future, it is best to test your final gravity with a hydrometer before bottling.