Here is a Simple way: Click your start button. Type %Appdata% In search (If you don't have Windows type Run and click Run on the type then type %Appdata% in there) Find .Minecraft then open it drag your Saves (And anything else you want to keep) to a safe place then Delete .Minecraft Then run Minecraft Then it should load all the Minecraft.jar files (Don't take The Minecraft.Jar sense this is usually caused by a Broken META-INF) Then go back to .Minecraft again and every thing you took out of the deleted one place in there. Run Minecraft and hopfully it will work. If not then play it one the launcher which you can download in the Download page. Hope this helped you.