if you aren't getting anything at all on the screen then the computer isn't even POSTing... which means there is something either disconnected or not connected properly.. such as Processor, RAM or IDE cables... or one of them is dead.
if you are getting something on the screen then something is failing which means there should be a message about the fail. if not, hit the delete key to go into bios to check your settings. if the HD failed, then it will probably give you a message teling you so, or at least it will go through POST