got the question mark folder flashing, held down. Shut down and restarted holding down Option key - the world came up and eventually mac os x utilities. When I click on Disk utility it I don't see my start up h.drive there. please help

Hi, I see others have had the same problem as me with the folder flashing with the question mark. I followed instructions holding down the option key on startup, got the world spinning all night long and now this morning it prompts me with the Mac OS X Utilities box. I clicked on Disk utility but the start up hard drive is not there - in only shows Mac OS X Base system and won't allow me to do anything. I click on Apple at top and go to Start up disk - and it is blank. PLEASE HELP.