Do you really expect an answer without giving any details about your computer configuration ? If your HDD is new then you need to install an OS first, if your HDD has an OS installed but was moved from an older computer it most likely won't boot as it tries to access the old chipset.
sorry, to be more specific i tried to install a new sata dvd-writer, but what happened is it that my ide hdd dint show in bios. so i disconnected the dvd- writer. now bios picks up the hdd again in bios, but it wont boot from the hdd no more. i checked boot order and it is set to boot from hdd. any ideas??