You might have to boot into Single User Mode by holding down Apple-S after the bootup chime. Usuall once you enter this mode, there are instructions right before you're dropped into the prompt. Do you get these same instructions when it goes into the command line mode by itself? If so, follow the instructions given. If not, then type "reboot", hit return, and right after the bootup chime hold down Apple-S and follow the instructions once at the prompt.