When you start your computer, you go immediately to the BIOS (touch Del, or F2, or F11, a brief message should indicate it at the bottom of the screen) and you verify if your hard disk is recognised. If it isn't, you will have to open your machine and verify if the cables are correctly wired. Ask to a friend, it may be hazardous.
PS : Excuse for my english. I don't use it very often.
No hay camino. Se hace camino al andar. (A. Machado)